diff --git a/src/lib/tz.ts b/src/lib/tz.ts
new file mode 100644
index 0000000..6f656a1
--- /dev/null
+++ b/src/lib/tz.ts
@@ -0,0 +1,32 @@
+import { fromZonedTime, toZonedTime, format as formatTz } from "date-fns-tz";
+
+/**
+ * Firm timezone — all calendar entries are entered and displayed in EST/EDT
+ * regardless of the browser's local timezone.
+ */
+export const FIRM_TZ = "America/New_York";
+
+/**
+ * Convert a "wall clock" date + HH:mm string entered in EST into an ISO UTC
+ * timestamp suitable for Postgres `timestamptz`. Handles DST automatically.
+ *
+ * Example: ("2024-07-04", "09:00") → "2024-07-04T13:00:00.000Z"  (EDT, -04:00)
+ *          ("2024-01-15", "09:00") → "2024-01-15T14:00:00.000Z"  (EST, -05:00)
+ */
+export function estWallTimeToUTC(dateStr: string, timeStr: string): string {
+  // Build a naive local-style string and tell date-fns-tz to interpret it as EST.
+  const naive = `${dateStr}T${timeStr.length === 5 ? `${timeStr}:00` : timeStr}`;
+  return fromZonedTime(naive, FIRM_TZ).toISOString();
+}
+
+/** Format a UTC ISO timestamp in EST using a date-fns format token. */
+export function formatInEST(iso: string | Date, fmt: string): string {
+  const d = typeof iso === "string" ? new Date(iso) : iso;
+  return formatTz(toZonedTime(d, FIRM_TZ), fmt, { timeZone: FIRM_TZ });
+}
+
+/** Get a Date object representing the EST wall-clock for a UTC ISO. */
+export function toESTDate(iso: string | Date): Date {
+  const d = typeof iso === "string" ? new Date(iso) : iso;
+  return toZonedTime(d, FIRM_TZ);
+}