diff --git a/apps/api/src/broker/vevent.ts b/apps/api/src/broker/vevent.ts
index ed2bf9c..7089e8f 100644
--- a/apps/api/src/broker/vevent.ts
+++ b/apps/api/src/broker/vevent.ts
@@ -80,8 +80,10 @@ export function buildVeventString(params: NewEventParams): { uid: string; icsStr
     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 })
+    // ICAL.Timezone.localTimezone is passed as the zone arg required by TS types.
+    // isDate:true suppresses any time/TZID output regardless of zone. (D-13)
+    const startTime = new ICAL.Time({ year: sy, month: sm, day: sd, isDate: true }, ICAL.Timezone.localTimezone)
+    const endTime = new ICAL.Time({ year: ey, month: em, day: ed, isDate: true }, ICAL.Timezone.localTimezone)
     vevent.addPropertyWithValue('dtstart', startTime)
     vevent.addPropertyWithValue('dtend', endTime)
   } else {
diff --git a/apps/api/src/broker/write.ts b/apps/api/src/broker/write.ts
new file mode 100644
index 0000000..4ab54da
--- /dev/null
+++ b/apps/api/src/broker/write.ts
@@ -0,0 +1,97 @@
+/**
+ * CalDAV write wrappers — broker boundary enforcement (D-12).
+ *
+ * This file is the ONLY place outside client.ts/sync.ts/poller.ts that issues
+ * PUT/DELETE to Fastmail via tsdav. Route handlers MUST NOT import tsdav directly
+ * (D-12 broker boundary). The outbox worker (outboxWorker.ts) calls these functions.
+ *
+ * T-03-04 (Spoofing / etag forgery): etag is sourced server-side from calendarEvents.etag
+ * by the worker — never accepted from the browser. write.ts only forwards what the
+ * server supplies.
+ *
+ * T-03-05 (Elevation of privilege): Calendar ownership is enforced at the route layer
+ * (Plan 04, V4). write.ts is a low-level primitive with no auth context.
+ *
+ * Status code interpretation is intentionally deferred to the caller (outboxWorker.ts):
+ *   - 201 / 204: success
+ *   - 412: conflict (D-08) → worker routes to conflict flow, no retry
+ *   - 400 / 401 / 403: hard fail → worker stops retrying immediately (D-07)
+ *   - 5xx / network error: transient → worker applies exponential backoff (D-07)
+ *
+ * Sources:
+ *   - https://tsdav.vercel.app/docs/caldav/createCalendarObject
+ *   - https://tsdav.vercel.app/docs/caldav/updateCalendarObject
+ *   - https://github.com/natelindev/tsdav/blob/main/src/request.ts (If-Match confirmed)
+ */
+
+import type { FastmailClient } from './client.js'
+import type { DAVCalendar } from 'tsdav'
+
+/**
+ * Creates a new CalDAV object via PUT with If-None-Match: * (create semantics).
+ *
+ * @param client      - Authenticated tsdav DAVClient
+ * @param calendar    - Target DAVCalendar (includes the calendar URL)
+ * @param uid         - Event UID; used as the object filename (`${uid}.ics`)
+ * @param icsString   - Full VCALENDAR/VEVENT string (from buildVeventString)
+ * @returns           Raw Response — caller interprets status codes
+ */
+export async function createCalendarEvent(
+  client: FastmailClient,
+  calendar: DAVCalendar,
+  uid: string,
+  icsString: string,
+): Promise<Response> {
+  return client.createCalendarObject({
+    calendar,
+    filename: `${uid}.ics`,
+    iCalString: icsString,
+  })
+}
+
+/**
+ * Updates an existing CalDAV object via PUT with If-Match: <etag> (D-08 conflict check).
+ *
+ * @param client             - Authenticated tsdav DAVClient
+ * @param calendarObjectUrl  - Full URL of the object (from calendarEvents.objectUrl)
+ * @param icsString          - Updated VCALENDAR/VEVENT string
+ * @param etag               - Cached etag from calendarEvents.etag; null is passed as ''
+ *                             (tsdav: etag → If-Match header; '' = no If-Match = unconditional)
+ * @returns                  Raw Response — 412 signals conflict (D-08)
+ */
+export async function updateCalendarEvent(
+  client: FastmailClient,
+  calendarObjectUrl: string,
+  icsString: string,
+  etag: string | null,
+): Promise<Response> {
+  return client.updateCalendarObject({
+    calendarObject: {
+      url: calendarObjectUrl,
+      data: icsString,
+      etag: etag ?? '', // tsdav maps etag → If-Match header; '' skips the header (safe default)
+    },
+  })
+}
+
+/**
+ * Deletes a CalDAV object via DELETE with If-Match: <etag> (D-08 conflict check).
+ *
+ * @param client             - Authenticated tsdav DAVClient
+ * @param calendarObjectUrl  - Full URL of the object (from calendarEvents.objectUrl)
+ * @param etag               - Cached etag from calendarEvents.etag; null is passed as ''
+ * @returns                  Raw Response — 412 signals conflict (D-08)
+ */
+export async function deleteCalendarEvent(
+  client: FastmailClient,
+  calendarObjectUrl: string,
+  etag: string | null,
+): Promise<Response> {
+  return client.deleteCalendarObject({
+    calendarObject: {
+      url: calendarObjectUrl,
+      data: '', // tsdav deleteCalendarObject requires the DAVCalendarObject shape; data unused
+      etag: etag ?? '',
+    },
+  })
+}