fix: keep the calendar day view working when midnight is skipped by DST

apps/mobile/src/date-time.ts

@@ -40,6 +40,29 @@ export function zonedInstant(date: string, time: string, timeZone: string): stri
   throw new Error("This time does not exist in the selected time zone. Choose another time.");
 }
 
+/**
+ * The first existing instant of a local day. Day-range boundaries are an internal
+ * computation, not a user-entered appointment time, so when a DST gap removes
+ * midnight (e.g. America/Santiago springs forward 00:00→01:00) the boundary
+ * clamps to the first existing local time instead of failing the whole query.
+ */
+export function startOfZonedDay(date: string, timeZone: string): string {
+  try {
+    return zonedInstant(date, "00:00", timeZone);
+  } catch (error) {
+    if (!(error instanceof Error) || !error.message.includes("does not exist")) throw error;
+  }
+  for (let minutes = 1; minutes < 24 * 60; minutes++) {
+    const time = `${String(Math.floor(minutes / 60)).padStart(2, "0")}:${String(minutes % 60).padStart(2, "0")}`;
+    try {
+      return zonedInstant(date, time, timeZone);
+    } catch {
+      // Still inside the gap; keep walking toward the first existing time.
+    }
+  }
+  throw new Error("This day has no existing local time in the selected time zone.");
+}
+
 /** Only fully serialized instants may reset a date editor's local text. */
 export function isCompleteInstant(value: string): boolean {
   return (

apps/mobile/src/screens.tsx

@@ -39,7 +39,7 @@ import type {
   EmailDraft,
 } from "../../../packages/domain/src";
 import { API_URL } from "./api";
-import { localDateTime, zonedInstant } from "./date-time";
+import { localDateTime, startOfZonedDay, zonedInstant } from "./date-time";
 import {
   Button,
   Card,
@@ -645,8 +645,8 @@ export function CalendarScreen() {
       .then(() => {
         const query = new URLSearchParams({
           calendarId,
-          timeMin: zonedInstant(date, "00:00", zone),
-          timeMax: zonedInstant(plusDays(date, all ? 30 : 1), "00:00", zone),
+          timeMin: startOfZonedDay(date, zone),
+          timeMax: startOfZonedDay(plusDays(date, all ? 30 : 1), zone),
         });
         return api.request<CalendarEvent[]>(`/api/calendar/events?${query}`);
       })

apps/mobile/test/date-time.test.ts

@@ -1,6 +1,11 @@
 import assert from "node:assert/strict";
 import test from "node:test";
-import { isCompleteInstant, localDateTime, zonedInstant } from "../src/date-time.ts";
+import {
+  isCompleteInstant,
+  localDateTime,
+  startOfZonedDay,
+  zonedInstant,
+} from "../src/date-time.ts";
 
 test("calendar time is rendered and entered in the selected named zone", () => {
   assert.deepEqual(localDateTime("2026-09-15T17:30:00Z", "America/Los_Angeles"), {
@@ -27,6 +32,14 @@ test("a daylight-saving gap cannot become a different appointment time", () => {
   );
 });
 
+test("a day boundary clamps to the first existing local time instead of failing", () => {
+  // America/Santiago springs forward 00:00→01:00 on 2026-09-06.
+  assert.equal(startOfZonedDay("2026-09-06", "America/Santiago"), "2026-09-06T04:00:00.000Z");
+  // Ordinary days still start exactly at midnight (Santiago is UTC-3 after the transition).
+  assert.equal(startOfZonedDay("2026-09-07", "America/Santiago"), "2026-09-07T03:00:00.000Z");
+  assert.equal(startOfZonedDay("2026-09-15", "UTC"), "2026-09-15T00:00:00.000Z");
+});
+
 test("partial native date edits never normalize from Date.parse", () => {
   for (const value of [
     "2026-09-1 09:00",