bug fix: Check offsets when browser doesn't canonicalize.

This is more of a workaround than a bug fix.

Some JS implementations are less "aggressive" about
canonicalizing things like America/Montreal to America/Toronto.

We instead use offset checks.

web/src/timerender.ts

@@ -555,18 +555,44 @@ export function should_display_profile_incomplete_alert(timestamp: number): bool
     return false;
 }
 
-export function browser_canonicalize_timezone(uncanonicalized_timezone: string): string {
+export function get_time_in_timezone(date: Date, timezone: string): number {
+    return Date.parse(date.toLocaleString("en-US", {timeZone: timezone}));
+}
+
+export function get_offset_difference_at_date(
+    timezone1: string,
+    timezone2: string,
+    reference_date: Date,
+): number {
+    const date1 = get_time_in_timezone(reference_date, timezone1);
+    const date2 = get_time_in_timezone(reference_date, timezone2);
+    return date1 - date2;
+}
+
+export function are_timezones_on_same_clock_now(timezone1: string, timezone2: string): boolean {
+    // America/Los_Angeles is clearly the same as America/Los_Angeles:
+    if (timezone1 === timezone2) {
+        return true;
+    }
+
+    // We still want this function to return true if the timezones are
+    // on the same clock for now, even though they may eventually diverge
+    // during Daylight Savings. This avoids nagging the user.  The only
+    // tradeoff is if the user stays logged on while the clocks change,
+    // but that should be rare.
+    const now = new Date();
+
     try {
-        // we use the runtime's default locale, to match _browser_time_zone_.
-        const canonicalized_timezone = new Intl.DateTimeFormat(undefined, {
-            timeZone: uncanonicalized_timezone,
-        }).resolvedOptions().timeZone;
-        return canonicalized_timezone;
+        return get_offset_difference_at_date(timezone1, timezone2, now) === 0;
     } catch {
-        return "";
+        // This should only happen during testing, but we just catch any error
+        // related to invalid time zones.
+        return false;
     }
 }
 
-export function is_browser_timezone_same_as(uncanonicalized_target_timezone: string): boolean {
-    return browser_time_zone() === browser_canonicalize_timezone(uncanonicalized_target_timezone);
+export function is_browser_timezone_same_as(zulip_time_zone: string): boolean {
+    // We delegate most of this check to facilitate testing.
+    // We don't want to mock browser_time_zone.
+    return are_timezones_on_same_clock_now(browser_time_zone(), zulip_time_zone);
 }

web/tests/timerender.test.cjs

@@ -654,6 +654,40 @@ run_test("should_display_profile_incomplete_alert", () => {
     assert.equal(timerender.should_display_profile_incomplete_alert(realm_date_created_secs), true);
 });
 
+run_test("are_timezones_on_same_clock_now", () => {
+    assert.ok(
+        timerender.are_timezones_on_same_clock_now("America/Los_Angeles", "America/Los_Angeles"),
+    );
+    assert.ok(timerender.are_timezones_on_same_clock_now("America/New_York", "America/New_York"));
+
+    // Montreal and Toronto are equivalent year-round.
+    assert.ok(timerender.are_timezones_on_same_clock_now("America/Montreal", "America/Toronto"));
+
+    assert.ok(
+        !timerender.are_timezones_on_same_clock_now("Invalid/Timezone", "America/Los_Angeles"),
+    );
+    assert.ok(
+        !timerender.are_timezones_on_same_clock_now("America/New_York", "America/Los_Angeles"),
+    );
+});
+
+run_test("are_timezones_on_same_clock_now (Phoenix)", () => {
+    /*
+        If you live in Phoenix and make the short flight to Los Angeles during
+        Daylight Savings time, you don't need to reset your watch.
+
+        Likewise, during the winter, if you go to Denver, you don't need to
+        reset your watch.
+
+        We err on the side of not nagging Phoenix folks to change their
+        "Zulip clock" for these short trips.
+    */
+    assert.ok(
+        timerender.are_timezones_on_same_clock_now("America/Phoenix", "America/Los_Angeles") ||
+            timerender.are_timezones_on_same_clock_now("America/Phoenix", "America/Denver"),
+    );
+});
+
 run_test("is_browser_timezone_same_as", () => {
     assert.equal(timerender.is_browser_timezone_same_as(timerender.browser_time_zone()), true);
 
@@ -661,19 +695,13 @@ run_test("is_browser_timezone_same_as", () => {
     assert.equal(timerender.is_browser_timezone_same_as("Invalid/Timezone"), false);
 });
 
-run_test("test time zone math", () => {
-    // This test doesn't seem to test any Zulip code, but it seems
-    // useful to keep around anyway, since the offset logic
-    // here is useful to understand.
-
+run_test("time zone helpers", () => {
     function get_time_in_timezone(date, timezone) {
-        return Date.parse(date.toLocaleString("en-US", {timeZone: timezone}));
+        return timerender.get_time_in_timezone(date, timezone);
     }
 
     function get_offset_difference_at_date(tz1, tz2, reference_date) {
-        const date1 = get_time_in_timezone(reference_date, tz1);
-        const date2 = get_time_in_timezone(reference_date, tz2);
-        return date1 - date2;
+        return timerender.get_offset_difference_at_date(tz1, tz2, reference_date);
     }
 
     // The current time in America/Phoenix does equal the current time