feat(events): add displayTimezone field and zoned datetime helpers

Events stored UTC have always been interpreted in viewer-local TZ at
the admin form and render layer. Adding an event-owned IANA timezone
unblocks accurate scheduling and display regardless of the admin's or
viewer's browser TZ.

- Event.displayTimezone (default "America/Toronto") on the model.
- displayTimezone added to admin create/update Zod schemas.
- app/utils/timezones.js: zonedLocalToUTC, utcToZonedLocal,
  shortTimezoneName — Intl-based helpers, no new dependencies.

app/utils/timezones.js

@@ -0,0 +1,77 @@
+// Convert a datetime-local string ("YYYY-MM-DDTHH:MM") to a UTC Date,
+// interpreting the wall-clock time in the given IANA timezone.
+export function zonedLocalToUTC(localStr, tz) {
+  if (!localStr || !tz) return null;
+  const [datePart, timePart] = String(localStr).split("T");
+  if (!datePart || !timePart) return null;
+  const [y, mo, d] = datePart.split("-").map(Number);
+  const [h, mi] = timePart.split(":").map(Number);
+  if ([y, mo, d, h, mi].some((n) => Number.isNaN(n))) return null;
+
+  // Treat the components as if they are already UTC. The result's wall-clock
+  // in the target TZ will differ from what we want by exactly the TZ offset
+  // for that moment, so we measure that offset and subtract it.
+  const asUTC = new Date(Date.UTC(y, mo - 1, d, h, mi));
+  const parts = new Intl.DateTimeFormat("en-US", {
+    timeZone: tz,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    hour: "2-digit",
+    minute: "2-digit",
+    second: "2-digit",
+    hour12: false,
+  }).formatToParts(asUTC);
+  const get = (type) => Number(parts.find((p) => p.type === type)?.value);
+  const observed = Date.UTC(
+    get("year"),
+    get("month") - 1,
+    get("day"),
+    get("hour") % 24,
+    get("minute"),
+    get("second"),
+  );
+  const offsetMs = observed - asUTC.getTime();
+  return new Date(asUTC.getTime() - offsetMs);
+}
+
+// Convert a UTC Date (or ISO string) to a datetime-local string
+// ("YYYY-MM-DDTHH:MM") rendered in the given IANA timezone.
+export function utcToZonedLocal(utc, tz) {
+  if (!utc || !tz) return "";
+  const d = utc instanceof Date ? utc : new Date(utc);
+  if (Number.isNaN(d.getTime())) return "";
+  const parts = new Intl.DateTimeFormat("en-US", {
+    timeZone: tz,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    hour: "2-digit",
+    minute: "2-digit",
+    hour12: false,
+  }).formatToParts(d);
+  const get = (type) => parts.find((p) => p.type === type)?.value;
+  const year = get("year");
+  const month = get("month");
+  const day = get("day");
+  let hour = get("hour");
+  const minute = get("minute");
+  if (hour === "24") hour = "00";
+  return `${year}-${month}-${day}T${hour}:${minute}`;
+}
+
+// Short timezone label (e.g., "EDT", "PDT") for a Date in a given IANA TZ.
+export function shortTimezoneName(date, tz) {
+  if (!date || !tz) return "";
+  const d = date instanceof Date ? date : new Date(date);
+  if (Number.isNaN(d.getTime())) return "";
+  try {
+    const parts = new Intl.DateTimeFormat("en-US", {
+      timeZone: tz,
+      timeZoneName: "short",
+    }).formatToParts(d);
+    return parts.find((p) => p.type === "timeZoneName")?.value || "";
+  } catch {
+    return "";
+  }
+}

server/models/event.js

@@ -18,6 +18,8 @@ const eventSchema = new mongoose.Schema({
     enum: ["community", "workshop", "social", "showcase"],
     default: "community",
   },
+  // IANA timezone for interpreting datetime input and rendering the event time.
+  displayTimezone: { type: String, default: "America/Toronto" },
   // Online-first location handling
   location: {
     type: String,

server/utils/schemas.js

@@ -173,6 +173,7 @@ export const adminEventCreateSchema = z.object({
   description: z.string().min(1).max(50000),
   startDate: z.string().min(1),
   endDate: z.string().min(1),
+  displayTimezone: z.string().max(100).optional(),
   location: z.string().max(500).optional(),
   maxAttendees: z.preprocess(emptyStringToUndefined, z.number().int().positive().optional().nullable()),
   membersOnly: z.boolean().optional(),
@@ -204,6 +205,7 @@ export const adminEventUpdateSchema = z.object({
   description: z.string().min(1).max(50000),
   startDate: z.string().min(1),
   endDate: z.string().min(1),
+  displayTimezone: z.string().max(100).optional(),
   location: z.string().max(500).optional(),
   maxAttendees: z.preprocess(emptyStringToUndefined, z.number().int().positive().optional().nullable()),
   membersOnly: z.boolean().optional(),