diff --git a/apps/api/src/broker/vevent.ts b/apps/api/src/broker/vevent.ts
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+++ b/apps/api/src/broker/vevent.ts
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+/**
+ * VEVENT builder — constructs a valid VCALENDAR/VEVENT iCalendar string for PUT to Fastmail.
+ *
+ * D-13 contract (mirrors sync.ts parser in reverse):
+ *   - All-day events: use ICAL.Time({ isDate: true }) → produces VALUE=DATE, no TZID, no time (D-13)
+ *   - Timed events:   use ICAL.Time.fromJSDate(date, true) (useUTC=true) → DTSTART:...Z suffix, no TZID
+ *   - NEVER coerce DATE to DATETIME (Pitfall #3)
+ *
+ * T-03-03 (Tampering): ical.js ICAL.Component handles line-folding + escaping (commas, semicolons,
+ * newlines in summary/location/description). Never hand-roll ICS strings.
+ *
+ * Sources:
+ *   - https://github.com/kewisch/ical.js/wiki/Convert-to-iCalendar-(rfc5545)
+ *   - https://github.com/kewisch/ical.js/blob/main/lib/ical/component.js
+ *   - https://github.com/kewisch/ical.js/blob/main/lib/ical/time.js
+ */
+
+import ICAL from 'ical.js'
+import { randomUUID } from 'crypto'
+
+export interface NewEventParams {
+  uid?: string          // omit = generate new UUID (appended with @familysync)
+  summary: string
+  allDay: boolean
+  // All-day: YYYY-MM-DD string (or Date — only the date portion is used)
+  // Timed: JS Date representing a UTC instant
+  dtstart: string | Date
+  dtend: string | Date
+  location?: string
+  description?: string
+  rruleString?: string  // e.g. 'FREQ=WEEKLY;BYDAY=MO' — omit for non-recurring (CAL-07)
+  dtstamp?: Date        // omit = now()
+}
+
+/**
+ * Simple RRULE preset strings for whole-series recurring events (D-11 / CAL-07).
+ * "weekly on Monday" example: 'FREQ=WEEKLY;BYDAY=MO' (not a key in this map; compose manually).
+ */
+export const RRULE_PRESETS: Record<string, string> = {
+  daily: 'FREQ=DAILY',
+  weekly: 'FREQ=WEEKLY',
+  monthly: 'FREQ=MONTHLY',
+  yearly: 'FREQ=YEARLY',
+}
+
+/**
+ * Builds a VCALENDAR/VEVENT iCalendar string from form parameters.
+ *
+ * @returns { uid, icsString } — uid is the generated or provided UID;
+ *   icsString is the full VCALENDAR string ready for PUT to Fastmail.
+ */
+export function buildVeventString(params: NewEventParams): { uid: string; icsString: string } {
+  const uid = params.uid ?? `${randomUUID()}@familysync`
+
+  // --- VCALENDAR wrapper ---
+  const cal = new ICAL.Component(['vcalendar', [], []])
+  cal.updatePropertyWithValue('version', '2.0')
+  cal.updatePropertyWithValue('prodid', '-//FamilySync//FamilySync//EN')
+
+  // --- VEVENT ---
+  const vevent = new ICAL.Component('vevent')
+  vevent.addPropertyWithValue('uid', uid)
+  vevent.addPropertyWithValue('summary', params.summary)
+
+  // DTSTAMP: always present (RFC 5545 §3.8.7.2 — required property)
+  const dtstamp = ICAL.Time.fromJSDate(params.dtstamp ?? new Date(), true)
+  vevent.addPropertyWithValue('dtstamp', dtstamp)
+
+  if (params.allDay) {
+    // DATE value (not DATETIME) — isDate:true produces VALUE=DATE, no time component (D-13 contract)
+    const startStr =
+      typeof params.dtstart === 'string'
+        ? params.dtstart
+        : params.dtstart.toISOString().slice(0, 10)
+    const endStr =
+      typeof params.dtend === 'string'
+        ? params.dtend
+        : params.dtend.toISOString().slice(0, 10)
+
+    const [sy, sm, sd] = startStr.split('-').map(Number) as [number, number, number]
+    const [ey, em, ed] = endStr.split('-').map(Number) as [number, number, number]
+
+    const startTime = new ICAL.Time({ year: sy, month: sm, day: sd, isDate: true })
+    const endTime = new ICAL.Time({ year: ey, month: em, day: ed, isDate: true })
+    vevent.addPropertyWithValue('dtstart', startTime)
+    vevent.addPropertyWithValue('dtend', endTime)
+  } else {
+    // DATETIME in UTC (useUTC=true → Z suffix; TZID is NOT added by ical.js) (D-13 contract)
+    const startTime = ICAL.Time.fromJSDate(params.dtstart as Date, true)
+    const endTime = ICAL.Time.fromJSDate(params.dtend as Date, true)
+    vevent.addPropertyWithValue('dtstart', startTime)
+    vevent.addPropertyWithValue('dtend', endTime)
+  }
+
+  // Optional: RRULE (CAL-07 — whole-series recurring events only in v1, D-11)
+  // Must use ICAL.Recur.fromString + ICAL.Property to get correct serialization.
+  // addPropertyWithValue('rrule', string) serializes the string character-by-character
+  // instead of as a RECUR value type.
+  if (params.rruleString) {
+    const recur = ICAL.Recur.fromString(params.rruleString)
+    const rruleProp = new ICAL.Property('rrule')
+    rruleProp.setValue(recur)
+    vevent.addProperty(rruleProp)
+  }
+
+  // Optional fields — included only when provided (no empty properties in ICS)
+  if (params.location) {
+    vevent.addPropertyWithValue('location', params.location)
+  }
+  if (params.description) {
+    vevent.addPropertyWithValue('description', params.description)
+  }
+
+  cal.addSubcomponent(vevent)
+
+  return { uid, icsString: cal.toString() }
+}