diff --git a/apps/hq-web/src/api.ts b/apps/hq-web/src/api.ts
index aa7f6bc..686c78a 100644
--- a/apps/hq-web/src/api.ts
+++ b/apps/hq-web/src/api.ts
@@ -196,6 +196,8 @@ export const patchClientModule = (id: string, body: Record<string, unknown>): Pr
   apiFetch<{ clientModule: ClientModule }>(`/client-modules/${id}`, { method: 'PATCH', body: JSON.stringify(body) })
     .then((r) => r.clientModule)
 
+export const getBillingSettings = (): Promise<{ paymentTermsDays: number }> =>
+  apiFetch<{ paymentTermsDays: number }>('/settings/billing')
 export const createDocument = (body: Record<string, unknown>): Promise<Doc> =>
   apiFetch<{ document: Doc }>('/documents', { method: 'POST', body: JSON.stringify(body) }).then((r) => r.document)
 export const getDocumentFull = (id: string): Promise<{ document: Doc; events: DocumentEvent[]; emails: EmailLogRow[]; shares: Share[] }> =>
diff --git a/apps/hq-web/src/pages/NewDocument.tsx b/apps/hq-web/src/pages/NewDocument.tsx
index 3c13687..a911851 100644
--- a/apps/hq-web/src/pages/NewDocument.tsx
+++ b/apps/hq-web/src/pages/NewDocument.tsx
@@ -3,7 +3,7 @@ import { useNavigate } from 'react-router-dom'
 import { formatINR, fromRupees } from '@sims/domain'
 import { Badge, Button, Field, Notice, PageHeader, Toolbar } from '@sims/ui'
 import {
-  createDocument, getClients, getModules, getPrices, previewDocument,
+  createDocument, getBillingSettings, getClients, getModules, getPrices, previewDocument,
   KIND_LABEL, type Client, type DocTotals, type Kind, type ModulePrice,
 } from '../api'
 import { LivePreview } from '../components/LivePreview'
@@ -66,6 +66,19 @@ export function NewDocument() {
   const [error, setError] = useState<string | undefined>()
   const [saving, setSaving] = useState(false)
 
+  // D18: invoice due date — auto-filled from billing.payment_terms_days, editable.
+  const [dueDate, setDueDate] = useState('')
+  const [termsDays, setTermsDays] = useState<number | undefined>()
+  useEffect(() => {
+    getBillingSettings().then((s) => setTermsDays(s.paymentTermsDays)).catch(() => { /* field stays manual */ })
+  }, [])
+  useEffect(() => {
+    if (docType === 'INVOICE' && dueDate === '' && termsDays !== undefined) {
+      const d = new Date(); d.setUTCDate(d.getUTCDate() + termsDays)
+      setDueDate(d.toISOString().slice(0, 10))
+    }
+  }, [docType, termsDays]) // eslint-disable-line react-hooks/exhaustive-deps -- prefill once, never clobber edits
+
   // -- live preview wiring: server totals + warnings replace all client money math --
   const [commitNonce, setCommitNonce] = useState(0)
   const commit = () => setCommitNonce((n) => n + 1)
@@ -112,8 +125,9 @@ export function NewDocument() {
       contentLines: splitContent(l.contentText),
     })),
     ...(terms.trim() !== '' ? { terms: terms.trim() } : {}),
+    ...(docType === 'INVOICE' && dueDate !== '' ? { dueDate } : {}),
   })
-  const previewBody = useMemo(() => JSON.stringify(buildDraftBody()), [docType, client, lines, terms])
+  const previewBody = useMemo(() => JSON.stringify(buildDraftBody()), [docType, client, lines, terms, dueDate])
 
   const save = () => {
     if (client === undefined) { setError('Pick a client first'); return }
@@ -192,6 +206,14 @@ export function NewDocument() {
           ))}
         </Toolbar>
       </Field>
+      {docType === 'INVOICE' && (
+        <Field label={`Due date${termsDays !== undefined ? ` (auto: +${termsDays}d)` : ''}`}>
+          <input
+            className="wf" type="date" value={dueDate}
+            onChange={(e) => { setDueDate(e.target.value); commit() }}
+          />
+        </Field>
+      )}
 
       <h3>Lines</h3>
       {lines.map((l, i) => {
diff --git a/apps/hq/src/api.ts b/apps/hq/src/api.ts
index e326632..9de75c5 100644
--- a/apps/hq/src/api.ts
+++ b/apps/hq/src/api.ts
@@ -429,7 +429,8 @@ export function apiRouter(
       const t = prepared.totals
       const doc: Doc = {
         id: 'preview', docType: input.docType, docNo: null, fy: fyOf(prepared.docDate),
-        clientId: client.id, docDate: prepared.docDate, status: 'draft', refDocId: null,
+        clientId: client.id, docDate: prepared.docDate, dueDate: prepared.dueDate ?? null,
+        status: 'draft', refDocId: null,
         taxablePaise: t.taxablePaise, cgstPaise: t.cgstPaise, sgstPaise: t.sgstPaise,
         igstPaise: t.igstPaise, roundOffPaise: t.roundOffPaise, payablePaise: t.payablePaise,
         payload: prepared.payload, source: 'preview', createdBy: staffId(res), createdAt: '',
diff --git a/apps/hq/src/db.ts b/apps/hq/src/db.ts
index 1841788..03246e7 100644
--- a/apps/hq/src/db.ts
+++ b/apps/hq/src/db.ts
@@ -53,6 +53,7 @@ CREATE TABLE IF NOT EXISTS document (
   id TEXT PRIMARY KEY, doc_type TEXT NOT NULL CHECK (doc_type IN
     ('QUOTATION','PROFORMA','INVOICE','RECEIPT','CREDIT_NOTE')),
   doc_no TEXT, fy TEXT NOT NULL, client_id TEXT NOT NULL, doc_date TEXT NOT NULL,
+  due_date TEXT, -- INVOICE only: stamped at issue from billing.payment_terms_days when absent (D18)
   status TEXT NOT NULL DEFAULT 'draft' CHECK (status IN
     ('draft','sent','accepted','invoiced','part_paid','paid','lost','cancelled')),
   ref_doc_id TEXT, -- QT→PI→INV chain; CN → the invoice it amends
@@ -187,6 +188,10 @@ function migrate(db: DB): void {
   if (!emailCols.some((c) => c.name === 'bounced')) {
     db.exec(`ALTER TABLE email_log ADD COLUMN bounced INTEGER NOT NULL DEFAULT 0`)
   }
+  const docCols = db.prepare(`PRAGMA table_info(document)`).all() as { name: string }[]
+  if (!docCols.some((c) => c.name === 'due_date')) {
+    db.exec(`ALTER TABLE document ADD COLUMN due_date TEXT`)
+  }
   const clientCols = db.prepare(`PRAGMA table_info(client)`).all() as { name: string }[]
   if (!clientCols.some((c) => c.name === 'owner_id')) {
     db.exec(`ALTER TABLE client ADD COLUMN owner_id TEXT`)
diff --git a/apps/hq/src/reminder-templates.ts b/apps/hq/src/reminder-templates.ts
index 80cf6b5..7d49fc8 100644
--- a/apps/hq/src/reminder-templates.ts
+++ b/apps/hq/src/reminder-templates.ts
@@ -36,12 +36,16 @@ export function reminderEmail(kind: ReminderRuleKind, ctx: ReminderContext): Rem
   switch (kind) {
     case 'invoice_overdue': {
       const amt = ctx.amountPaise !== undefined ? formatINR(ctx.amountPaise) : ''
+      // D18: with a due date the mail says the honest thing ("was due on X, N days
+      // overdue"); legacy invoices without one keep the past-due-since-issue wording.
+      const lateness = ctx.dueDate !== undefined
+        ? ` was due on ${ctx.dueDate}` + (ctx.daysOverdue !== undefined ? ` and is now ${ctx.daysOverdue} day(s) overdue` : '')
+        : ` is outstanding` + (ctx.daysOverdue !== undefined ? ` and is now ${ctx.daysOverdue} day(s) past due` : '')
       return {
         subject: `Payment reminder — Invoice ${ctx.docNo} (${ctx.companyName})`,
         bodyText:
           `Dear ${ctx.clientName},\n\n`
-          + `This is a gentle reminder that Invoice ${ctx.docNo}${amt !== '' ? ` for ${amt}` : ''} is outstanding`
-          + (ctx.daysOverdue !== undefined ? ` and is now ${ctx.daysOverdue} day(s) past due` : '')
+          + `This is a gentle reminder that Invoice ${ctx.docNo}${amt !== '' ? ` for ${amt}` : ''}${lateness}`
           + `. We would be grateful if you could arrange payment at your earliest convenience.\n\n`
           + `If payment has already been made, kindly ignore this message.\n\n`
           + signOff(ctx),
diff --git a/apps/hq/src/repos-documents.ts b/apps/hq/src/repos-documents.ts
index ab26454..b186e8b 100644
--- a/apps/hq/src/repos-documents.ts
+++ b/apps/hq/src/repos-documents.ts
@@ -4,7 +4,7 @@ import { writeAudit } from './audit'
 import type { DB } from './db'
 import { getClient } from './repos-clients'
 import { getModule, priceOn, type Kind } from './repos-modules'
-import { dismissQuoteFollowups } from './repos-reminders'
+import { dismissQuoteFollowups, getNumberSetting } from './repos-reminders'
 import { nextDocNo } from './series'
 
 /**
@@ -36,7 +36,7 @@ export interface DocPayload {
 
 export interface Doc {
   id: string; docType: DocType; docNo: string | null; fy: string; clientId: string
-  docDate: string; status: DocStatus; refDocId: string | null
+  docDate: string; dueDate: string | null; status: DocStatus; refDocId: string | null
   taxablePaise: number; cgstPaise: number; sgstPaise: number; igstPaise: number
   roundOffPaise: number; payablePaise: number
   payload: DocPayload; source: string; createdBy: string; createdAt: string
@@ -44,7 +44,7 @@ export interface Doc {
 
 interface DocRow {
   id: string; doc_type: string; doc_no: string | null; fy: string; client_id: string
-  doc_date: string; status: string; ref_doc_id: string | null
+  doc_date: string; due_date: string | null; status: string; ref_doc_id: string | null
   taxable_paise: number; cgst_paise: number; sgst_paise: number; igst_paise: number
   round_off_paise: number; payable_paise: number
   payload: string; source: string; created_by: string; created_at: string
@@ -53,7 +53,8 @@ interface DocRow {
 function toDoc(r: DocRow): Doc {
   return {
     id: r.id, docType: r.doc_type as DocType, docNo: r.doc_no, fy: r.fy,
-    clientId: r.client_id, docDate: r.doc_date, status: r.status as DocStatus,
+    clientId: r.client_id, docDate: r.doc_date, dueDate: r.due_date,
+    status: r.status as DocStatus,
     refDocId: r.ref_doc_id,
     taxablePaise: r.taxable_paise, cgstPaise: r.cgst_paise, sgstPaise: r.sgst_paise,
     igstPaise: r.igst_paise, roundOffPaise: r.round_off_paise, payablePaise: r.payable_paise,
@@ -117,10 +118,18 @@ export interface DraftInput {
   clientId: string
   lines: DraftLineInput[]
   terms?: string
+  /** INVOICE only (D18): payment due date; when absent, issue stamps doc_date + terms. */
+  dueDate?: string
 }
 
 const todayIso = (): string => new Date().toISOString().slice(0, 10)
 
+/** YYYY-MM-DD + N days (UTC). Local copy — importing scheduler's helper would cycle. */
+const addDays = (dateIso: string, days: number): string => {
+  const [y, m, d] = dateIso.split('-').map(Number)
+  return new Date(Date.UTC(y!, m! - 1, d! + days)).toISOString().slice(0, 10)
+}
+
 function supplyStateCode(db: DB): string {
   const row = db.prepare(`SELECT value FROM setting WHERE key='company.state_code'`)
     .get() as { value: string } | undefined
@@ -175,17 +184,17 @@ function buildLines(db: DB, inputs: DraftLineInput[], onDate: string, warnings?:
 /** Insert a draft row + created event + audit. Callers wrap in a transaction. */
 function insertDocRow(db: DB, userId: string, a: {
   docType: DocType; clientId: string; refDocId: string | null; docDate: string
-  totals: BillTotals; payload: DocPayload
+  totals: BillTotals; payload: DocPayload; dueDate?: string | null
 }): Doc {
   const id = uuidv7()
   const t = a.totals
   db.prepare(
-    `INSERT INTO document (id, doc_type, doc_no, fy, client_id, doc_date, status, ref_doc_id,
+    `INSERT INTO document (id, doc_type, doc_no, fy, client_id, doc_date, due_date, status, ref_doc_id,
        taxable_paise, cgst_paise, sgst_paise, igst_paise, round_off_paise, payable_paise,
        payload, created_by, created_at)
-     VALUES (?, ?, NULL, ?, ?, ?, 'draft', ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
+     VALUES (?, ?, NULL, ?, ?, ?, ?, 'draft', ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)`,
   ).run(
-    id, a.docType, fyOf(a.docDate), a.clientId, a.docDate, a.refDocId,
+    id, a.docType, fyOf(a.docDate), a.clientId, a.docDate, a.dueDate ?? null, a.refDocId,
     t.taxablePaise, t.cgstPaise, t.sgstPaise, t.igstPaise, t.roundOffPaise, t.payablePaise,
     JSON.stringify(a.payload), userId, new Date().toISOString(),
   )
@@ -209,6 +218,7 @@ function zeroTotals(): BillTotals {
 export interface PreparedDraft {
   docType: DocType; clientId: string; docDate: string
   totals: BillTotals; payload: DocPayload; warnings: string[]
+  dueDate?: string
 }
 
 /**
@@ -223,6 +233,11 @@ export function prepareDraft(db: DB, input: DraftInput, opts: { permissive?: boo
   if (!['QUOTATION', 'PROFORMA', 'INVOICE'].includes(input.docType)) {
     throw new Error(`Cannot draft doc type: ${input.docType}`)
   }
+  // Due dates are payment terms — they belong to invoices only (D18, spec A2).
+  if (input.dueDate !== undefined) {
+    if (input.docType !== 'INVOICE') throw new Error('Due date applies to INVOICE drafts only')
+    if (!/^\d{4}-\d{2}-\d{2}$/.test(input.dueDate)) throw new Error('dueDate must be YYYY-MM-DD')
+  }
   const client = getClient(db, input.clientId)
   if (client === null && !permissive) throw new Error('Client not found')
   const today = todayIso()
@@ -254,6 +269,7 @@ export function prepareDraft(db: DB, input: DraftInput, opts: { permissive?: boo
   return {
     docType: input.docType, clientId: input.clientId, docDate: today,
     totals: computed.totals, payload, warnings,
+    ...(input.dueDate !== undefined ? { dueDate: input.dueDate } : {}),
   }
 }
 
@@ -262,6 +278,7 @@ export function createDraft(db: DB, userId: string, input: DraftInput): Doc {
   return db.transaction(() => insertDocRow(db, userId, {
     docType: prepared.docType, clientId: prepared.clientId, refDocId: null,
     docDate: prepared.docDate, totals: prepared.totals, payload: prepared.payload,
+    dueDate: prepared.dueDate ?? null,
   }))()
 }
 
@@ -276,8 +293,16 @@ export function issueDocument(db: DB, userId: string, docId: string): Doc {
     if (before.status === 'cancelled') throw new Error('Cannot issue a cancelled document')
     const docNo = nextDocNo(db, before.docType, before.fy)
     db.prepare(`UPDATE document SET doc_no=? WHERE id=?`).run(docNo, docId)
+    // D18: an invoice that reaches issue without a due date gets doc_date + terms
+    // stamped NOW — the last moment it is still a draft (issued paper never changes).
+    let dueDate = before.dueDate
+    if (before.docType === 'INVOICE' && dueDate === null) {
+      const terms = getNumberSetting(db, 'billing.payment_terms_days', 15)
+      dueDate = addDays(before.docDate, terms)
+      db.prepare(`UPDATE document SET due_date=? WHERE id=?`).run(dueDate, docId)
+    }
     addEvent(db, docId, 'issued', { docNo })
-    writeAudit(db, userId, 'issue', 'document', docId, { docNo: null }, { docNo })
+    writeAudit(db, userId, 'issue', 'document', docId, { docNo: null }, { docNo, ...(dueDate !== before.dueDate ? { dueDate } : {}) })
     return getDocument(db, docId)!
   })()
 }
diff --git a/apps/hq/src/scheduler.ts b/apps/hq/src/scheduler.ts
index 56d7f56..43f6c95 100644
--- a/apps/hq/src/scheduler.ts
+++ b/apps/hq/src/scheduler.ts
@@ -112,21 +112,23 @@ export async function runDailyScan(db: DB, deps: ScanDeps, today: string): Promi
   const now = deps.now?.() ?? new Date().toISOString()
   const created: Record<string, number> = {}
 
-  // --- invoice_overdue (dN day-past-issue milestones from the dated schedule; spec §8) ---
-  // A7: there is no due-date column — age anchors on doc_date (the issue date).
+  // --- invoice_overdue (dN day-past-due milestones from the dated schedule; spec §8, D18) ---
+  // Age anchors on the due date when the invoice carries one (stamped at issue from
+  // billing.payment_terms_days), else the issue date — the pre-D18 behavior.
   // Catch-up guard (F7): only the HIGHEST crossed milestone enqueues per scan, so a
   // scan gap (or the old monthly→dN cutover) nudges once, never the whole ladder;
   // under daily scans each milestone still fires exactly once as it is crossed.
   const invOffsets = resolveSchedule(db, 'invoice_overdue', today).dayOffsets // ascending
   const overdueCutoff = addDaysIso(today, -invOffsets[0]!)
   const overdue = db.prepare(
-    `SELECT id, client_id, doc_date FROM document
+    `SELECT id, client_id, doc_date, due_date FROM document
      WHERE doc_type='INVOICE' AND doc_no IS NOT NULL
-       AND status NOT IN ('paid','cancelled','lost') AND doc_date <= ?`,
-  ).all(overdueCutoff) as { id: string; client_id: string; doc_date: string }[]
+       AND status NOT IN ('paid','cancelled','lost')
+       AND COALESCE(due_date, doc_date) <= ?`,
+  ).all(overdueCutoff) as { id: string; client_id: string; doc_date: string; due_date: string | null }[]
   for (const inv of overdue) {
     if (outstandingPaise(db, inv.id) <= 0) continue
-    const age = daysBetweenIso(inv.doc_date, today)
+    const age = daysBetweenIso(inv.due_date ?? inv.doc_date, today)
     const crossed = invOffsets.filter((d) => age >= d)
     if (crossed.length === 0) continue
     const highest = crossed[crossed.length - 1]!
diff --git a/apps/hq/src/seed.ts b/apps/hq/src/seed.ts
index 576d66d..77c1462 100644
--- a/apps/hq/src/seed.ts
+++ b/apps/hq/src/seed.ts
@@ -70,6 +70,13 @@ export function seedIfEmpty(db: DB): void {
   for (const [key, value] of Object.entries(REMINDER_SETTINGS)) seededReminderSettings += insert.run(key, value).changes
   if (seededReminderSettings > 0) writeAudit(db, 'system', 'seed', 'setting', 'reminders.*', undefined, REMINDER_SETTINGS)
 
+  // Billing defaults (D18): payment terms in days — issueDocument stamps
+  // due_date = doc_date + terms on invoices that don't carry one.
+  const BILLING_SETTINGS: Record<string, string> = { 'billing.payment_terms_days': '15' }
+  let seededBilling = 0
+  for (const [key, value] of Object.entries(BILLING_SETTINGS)) seededBilling += insert.run(key, value).changes
+  if (seededBilling > 0) writeAudit(db, 'system', 'seed', 'setting', 'billing.*', undefined, BILLING_SETTINGS)
+
   // Dated reminder schedules (rule 3 / D-REMIND): one open-ended row per rule kind,
   // matching the code-constant fallback. A cadence change is a NEW dated row, not an edit.
   const scheduleKinds: ScheduleRuleKind[] = ['quote_followup', 'invoice_overdue']
diff --git a/apps/hq/src/send-reminder.ts b/apps/hq/src/send-reminder.ts
index 48cf4be..a0ae429 100644
--- a/apps/hq/src/send-reminder.ts
+++ b/apps/hq/src/send-reminder.ts
@@ -44,15 +44,18 @@ export function reminderContext(db: DB, reminder: Reminder, companyName: string,
     ctx.amountPaise = doc.payablePaise
     ctx.period = reminder.duePeriod
     if (reminder.ruleKind === 'invoice_overdue') {
-      // A7: anchored on the invoice's issue date (no due-date column) — renders the
-      // template's "N day(s) past due". `today` injectable like the quote branch below.
+      // D18: anchored on the due date when the invoice carries one (stamped at issue),
+      // else the issue date (legacy invoices) — renders "N day(s) past due"/"overdue".
+      // `today` injectable like the quote branch below.
       // (Local day math: importing scheduler's helper here would create an import cycle.)
       const onDate = today ?? new Date().toISOString().slice(0, 10)
       const atUtc = (s: string): number => {
         const [y, m, d] = s.split('-').map(Number)
         return Date.UTC(y!, m! - 1, d!)
       }
-      ctx.daysOverdue = Math.max(0, Math.round((atUtc(onDate) - atUtc(doc.docDate)) / 86_400_000))
+      const anchor = doc.dueDate ?? doc.docDate
+      if (doc.dueDate !== null) ctx.dueDate = doc.dueDate
+      ctx.daysOverdue = Math.max(0, Math.round((atUtc(onDate) - atUtc(anchor)) / 86_400_000))
     }
     if (reminder.ruleKind === 'quote_followup') {
       // `today` is injectable (deps.now via sendReminder) so the send path resolves the
diff --git a/apps/hq/src/templates.ts b/apps/hq/src/templates.ts
index 5e7d3aa..f2d48c9 100644
--- a/apps/hq/src/templates.ts
+++ b/apps/hq/src/templates.ts
@@ -199,6 +199,7 @@ export function documentHtml(doc: Doc, client: Client, company: Record<string, s
     <div class="docmeta">
       <div class="row"><span class="k">Document No</span><span class="v">${doc.docNo !== null ? esc(doc.docNo) : '<span class="chip-draft">DRAFT</span>'}</span></div>
       <div class="row"><span class="k">Date</span><span class="v">${displayDate(doc.docDate)}</span></div>
+      ${doc.docType === 'INVOICE' && typeof doc.dueDate === 'string' ? `<div class="row"><span class="k">Due Date</span><span class="v">${displayDate(doc.dueDate)}</span></div>` : ''}
       <div class="row"><span class="k">FY</span><span class="v">${esc(doc.fy)}</span></div>
     </div>
   </section>
@@ -353,7 +354,7 @@ export function documentHtmlSample(company: Record<string, string>, opts: { cont
   }
   const doc: Doc = {
     id: 'sample', docType: 'INVOICE', docNo: null, fy: '2026-27', clientId: 'sample',
-    docDate: new Date().toISOString().slice(0, 10), status: 'draft', refDocId: null,
+    docDate: new Date().toISOString().slice(0, 10), dueDate: null, status: 'draft', refDocId: null,
     taxablePaise: 10_000_00, cgstPaise: 900_00, sgstPaise: 900_00, igstPaise: 0,
     roundOffPaise: 0, payablePaise: 11_800_00,
     payload: {
diff --git a/apps/hq/test/dashboard.test.ts b/apps/hq/test/dashboard.test.ts
index 66870a7..9e1809a 100644
--- a/apps/hq/test/dashboard.test.ts
+++ b/apps/hq/test/dashboard.test.ts
@@ -23,7 +23,7 @@ function seeded() {
   const m = createModule(db, 'u1', { code: 'POS', name: 'POS' })
   setPrice(db, 'u1', { moduleId: m.id, kind: 'yearly', pricePaise: 10_000_00, effectiveFrom: '2026-01-01' })
   const inv = issueDocument(db, 'u1', createDraft(db, 'u1', { docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }] }).id)
-  db.prepare(`UPDATE document SET doc_date='2026-06-01' WHERE id=?`).run(inv.id)
+  db.prepare(`UPDATE document SET doc_date='2026-06-01', due_date=NULL WHERE id=?`).run(inv.id)
   const cm = assignModule(db, 'u1', { clientId: c.id, moduleId: m.id, kind: 'yearly' })
   updateClientModule(db, 'u1', cm.id, { nextRenewal: '2026-07-20' })
   createInteraction(db, 'u1', { clientId: c.id, typeCode: 'call', onDate: '2026-07-01', followUpOn: '2026-07-09' })
diff --git a/apps/hq/test/due-dates.test.ts b/apps/hq/test/due-dates.test.ts
new file mode 100644
index 0000000..c105506
--- /dev/null
+++ b/apps/hq/test/due-dates.test.ts
@@ -0,0 +1,122 @@
+// apps/hq/test/due-dates.test.ts — go-live cluster WS-A: invoice due dates (D18)
+import { describe, it, expect } from 'vitest'
+import { openDb } from '../src/db'
+import { seedIfEmpty } from '../src/seed'
+import { createClient } from '../src/repos-clients'
+import { createModule, setPrice } from '../src/repos-modules'
+import { convertDocument, createDraft, getDocument, issueDocument } from '../src/repos-documents'
+import { getReminder, listReminders, setSetting, upsertReminder } from '../src/repos-reminders'
+import { reminderContext } from '../src/send-reminder'
+import { reminderEmail } from '../src/reminder-templates'
+import { runDailyScan, type ScanDeps } from '../src/scheduler'
+
+const deps: ScanDeps = {
+  gmail: { f: (async () => new Response('{}')) as typeof fetch, clientId: '', clientSecret: '', keyHex: '' },
+  renderPdf: async () => Buffer.from('%PDF-'), company: () => ({ 'company.name': 'Tecnostac' }),
+  now: () => '2026-07-17T00:00:00Z',
+}
+
+function world() {
+  const db = openDb(':memory:'); seedIfEmpty(db) // seeds billing.payment_terms_days=15
+  const c = createClient(db, 'u1', { name: 'Acme', stateCode: '32', contacts: [{ name: 'R', email: 'r@acme.in' }] })
+  const m = createModule(db, 'u1', { code: 'POS', name: 'POS' })
+  setPrice(db, 'u1', { moduleId: m.id, kind: 'yearly', pricePaise: 10_000_00, effectiveFrom: '2026-01-01' })
+  const invoiceDraft = () => createDraft(db, 'u1', {
+    docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }],
+  })
+  return { db, c, m, invoiceDraft }
+}
+
+describe('due dates (D18)', () => {
+  it('issue stamps doc_date + terms on an invoice without one (manual issue path)', () => {
+    const { db, invoiceDraft } = world()
+    const draft = invoiceDraft()
+    expect(draft.dueDate).toBeNull()
+    db.prepare(`UPDATE document SET doc_date='2026-07-01' WHERE id=?`).run(draft.id)
+    const inv = issueDocument(db, 'u1', draft.id)
+    expect(inv.dueDate).toBe('2026-07-16') // 2026-07-01 + 15
+  })
+
+  it('a caller-provided due date survives issue untouched', () => {
+    const { db, c, m } = world()
+    const draft = createDraft(db, 'u1', {
+      docType: 'INVOICE', clientId: c.id, dueDate: '2026-09-30',
+      lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }],
+    })
+    expect(draft.dueDate).toBe('2026-09-30')
+    expect(issueDocument(db, 'u1', draft.id).dueDate).toBe('2026-09-30')
+  })
+
+  it('the terms setting is honoured when changed (dated behavior lives in the stamp)', () => {
+    const { db, invoiceDraft } = world()
+    setSetting(db, 'u1', 'billing.payment_terms_days', '30')
+    const draft = invoiceDraft()
+    db.prepare(`UPDATE document SET doc_date='2026-07-01' WHERE id=?`).run(draft.id)
+    expect(issueDocument(db, 'u1', draft.id).dueDate).toBe('2026-07-31')
+  })
+
+  it('quotes and proformas never get a due date — at draft or at issue', () => {
+    const { db, c, m } = world()
+    expect(() => createDraft(db, 'u1', {
+      docType: 'QUOTATION', clientId: c.id, dueDate: '2026-08-01',
+      lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }],
+    })).toThrow(/INVOICE/)
+    const pi = createDraft(db, 'u1', {
+      docType: 'PROFORMA', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }],
+    })
+    expect(issueDocument(db, 'u1', pi.id).dueDate).toBeNull()
+  })
+
+  it('proforma → invoice conversion gets stamped at issue (convert-and-send path)', () => {
+    const { db, c, m } = world()
+    const pi = issueDocument(db, 'u1', createDraft(db, 'u1', {
+      docType: 'PROFORMA', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }],
+    }).id)
+    const invDraft = convertDocument(db, 'u1', pi.id, 'INVOICE')
+    const inv = issueDocument(db, 'u1', invDraft.id)
+    expect(inv.dueDate).not.toBeNull() // stamped from terms on the invoice's own doc_date
+  })
+
+  it('bad dueDate format is rejected at draft', () => {
+    const { db, c, m } = world()
+    expect(() => createDraft(db, 'u1', {
+      docType: 'INVOICE', clientId: c.id, dueDate: '17-07-2026',
+      lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }],
+    })).toThrow(/YYYY-MM-DD/)
+  })
+
+  it('overdue scan anchors on the due date: not yet due ⇒ silent, past due ⇒ milestone from due_date', async () => {
+    const { db, invoiceDraft } = world()
+    // Old issue date but due date in the future → NOT overdue.
+    const a = issueDocument(db, 'u1', invoiceDraft().id)
+    db.prepare(`UPDATE document SET doc_date='2026-05-01', due_date='2026-08-01' WHERE id=?`).run(a.id)
+    // Due 16 days ago → d15 milestone (7/15/30 ladder), counted from due_date not doc_date.
+    const b = issueDocument(db, 'u1', invoiceDraft().id)
+    db.prepare(`UPDATE document SET doc_date='2026-05-01', due_date='2026-07-01' WHERE id=?`).run(b.id)
+    await runDailyScan(db, deps, '2026-07-17')
+    const rows = listReminders(db, {}).filter((r) => r.ruleKind === 'invoice_overdue')
+    expect(rows).toHaveLength(1)
+    expect(rows[0]).toMatchObject({ subjectId: b.id, duePeriod: 'd15' })
+  })
+
+  it('the reminder email says "was due on X and is now N day(s) overdue"', () => {
+    const { db, c, invoiceDraft } = world()
+    const inv = issueDocument(db, 'u1', invoiceDraft().id)
+    db.prepare(`UPDATE document SET due_date='2026-07-02' WHERE id=?`).run(inv.id)
+    const { id } = upsertReminder(db, { ruleKind: 'invoice_overdue', subjectId: inv.id, duePeriod: 'd15', clientId: c.id, docId: inv.id, now: '2026-07-17T00:00:00Z' })
+    const { ctx } = reminderContext(db, getReminder(db, id)!, 'Tecnostac', '2026-07-17')
+    expect(ctx.dueDate).toBe('2026-07-02')
+    expect(ctx.daysOverdue).toBe(15)
+    expect(reminderEmail('invoice_overdue', ctx).bodyText).toMatch(/was due on 2026-07-02 and is now 15 day\(s\) overdue/)
+  })
+
+  it('legacy invoices without a due date keep the past-due-since-issue wording and doc_date anchor', () => {
+    const { db, c, invoiceDraft } = world()
+    const inv = issueDocument(db, 'u1', invoiceDraft().id)
+    db.prepare(`UPDATE document SET doc_date='2026-07-07', due_date=NULL WHERE id=?`).run(inv.id)
+    const { id } = upsertReminder(db, { ruleKind: 'invoice_overdue', subjectId: inv.id, duePeriod: 'd7', clientId: c.id, docId: inv.id, now: '2026-07-17T00:00:00Z' })
+    const { ctx } = reminderContext(db, getReminder(db, id)!, 'Tecnostac', '2026-07-17')
+    expect(ctx.dueDate).toBeUndefined()
+    expect(reminderEmail('invoice_overdue', ctx).bodyText).toMatch(/is outstanding and is now 10 day\(s\) past due/)
+  })
+})
diff --git a/apps/hq/test/scheduler-scan.test.ts b/apps/hq/test/scheduler-scan.test.ts
index 1ab32b1..eb8da6f 100644
--- a/apps/hq/test/scheduler-scan.test.ts
+++ b/apps/hq/test/scheduler-scan.test.ts
@@ -31,7 +31,7 @@ describe('runDailyScan — detection rules', () => {
     const { db, c, m } = world()
     // Overdue invoice: issued 2026-06-01, unpaid, > 7 days before 2026-07-10.
     const inv = issueDocument(db, 'u1', createDraft(db, 'u1', { docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }] }).id)
-    db.prepare(`UPDATE document SET doc_date='2026-06-01' WHERE id=?`).run(inv.id)
+    db.prepare(`UPDATE document SET doc_date='2026-06-01', due_date=NULL WHERE id=?`).run(inv.id)
     // Renewal within 15 days.
     const cm = assignModule(db, 'u1', { clientId: c.id, moduleId: m.id, kind: 'yearly' })
     updateClientModule(db, 'u1', cm.id, { nextRenewal: '2026-07-20' })
@@ -55,7 +55,7 @@ describe('runDailyScan — detection rules', () => {
   it('does not raise a reminder for an invoice that is not yet overdue', async () => {
     const { db, c, m } = world()
     const inv = issueDocument(db, 'u1', createDraft(db, 'u1', { docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }] }).id)
-    db.prepare(`UPDATE document SET doc_date='2026-07-08' WHERE id=?`).run(inv.id) // only 2 days old
+    db.prepare(`UPDATE document SET doc_date='2026-07-08', due_date=NULL WHERE id=?`).run(inv.id) // only 2 days old
     const res = await runDailyScan(db, deps, '2026-07-10')
     expect(res.created['invoice_overdue'] ?? 0).toBe(0)
   })
@@ -67,7 +67,7 @@ describe('invoice_overdue — dN milestone escalation (Phase 9, spec §8)', () =
     const inv = issueDocument(db, 'u1', createDraft(db, 'u1', {
       docType: 'INVOICE', clientId: c.id, lines: [{ moduleId: m.id, qty: 1, kind: 'yearly' }],
     }).id)
-    db.prepare(`UPDATE document SET doc_date=? WHERE id=?`).run(docDate, inv.id)
+    db.prepare(`UPDATE document SET doc_date=?, due_date=NULL WHERE id=?`).run(docDate, inv.id)
     return { db, c, inv }
   }
   const invoiceRows = (db: ReturnType<typeof openDb>) =>