fix(date): %s returns Unix epoch unaffected by display timezone (#932)

The %s handler read LiquidDate.getTime(), which returns the
displayDate deliberately shifted by the display timezone offset for
wall-clock getters. With a timezone argument or timezoneOffset
option set, %s produced an epoch shifted by (server offset - display
offset) instead of the true Unix timestamp.

Expose the unshifted time as LiquidDate.dateValue() and use it for
%s. Also switch Math.round to Math.floor so fractional seconds
truncate toward the epoch like Ruby strftime.

Fixes #931

docs/source/filters/date.md

@@ -34,6 +34,7 @@ The `date` filter is used to convert a timestamp into the specified format.
             * minutes: `-360` means `'+06:00'` and `360` means `'-06:00'`
             * timeZone ID: `Asia/Colombo` or `America/New_York`
     * See [here](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) for TZ database values
+* `%s` (seconds since the Unix epoch) identifies an instant rather than a wall-clock time, so it's not affected by the display timezone.
 
 ### Examples
 ```liquid

src/util/liquid-date.ts

@@ -53,6 +53,15 @@ export class LiquidDate {
   getTime () {
     return this.displayDate.getTime()
   }
+  /**
+   * The underlying UTC timestamp in milliseconds, unaffected by the display
+   * timezone. Use this (not `getTime()`) for timezone-invariant values like
+   * `%s`: `getTime()` reads `displayDate`, which is deliberately shifted by
+   * the display timezone offset so wall-clock getters can delegate to Date.
+   */
+  dateValue () {
+    return this.date.getTime()
+  }
   getMilliseconds () {
     return this.displayDate.getMilliseconds()
   }

src/util/strftime.ts

@@ -105,7 +105,7 @@ const formatCodes: Record<string, FormatCodeHandler> = {
   p: (d: LiquidDate) => (d.getHours() < 12 ? 'AM' : 'PM'),
   P: (d: LiquidDate) => (d.getHours() < 12 ? 'am' : 'pm'),
   q: (d: LiquidDate) => ordinal(d),
-  s: (d: LiquidDate) => Math.round(d.getTime() / 1000),
+  s: (d: LiquidDate) => Math.floor(d.dateValue() / 1000),
   S: (d: LiquidDate) => d.getSeconds(),
   u: (d: LiquidDate) => d.getDay() || 7,
   U: (d: LiquidDate) => getWeekOfYear(d, 0),

test/integration/filters/date.spec.ts

@@ -140,6 +140,18 @@ describe('filters/date', function () {
     it('should support timezone name argument when DST is active', function () {
       return test('{{ "2021-06-01T23:00:00Z" | date: "%Y-%m-%dT%H:%M:%S", "America/New_York" }}', '2021-06-01T19:00:00')
     })
+    it('should not shift %s by the timezone name argument', function () {
+      return test('{{ "2026-06-30T21:00:00Z" | date: "%s", "America/Toronto" }}', '1782853200')
+    })
+    it('should not shift %s by the timezone offset argument', function () {
+      return test('{{ "2026-06-30T21:00:00Z" | date: "%s", 360 }}', '1782853200')
+    })
+    it('should not shift %s by the timezoneOffset option', function () {
+      return test('{{ "2026-06-30T21:00:00Z" | date: "%s" }}', '1782853200', undefined, opts)
+    })
+    it('should truncate %s toward the epoch like Ruby strftime', function () {
+      return test('{{ "2026-06-30T17:00:00.500Z" | date: "%s" }}', '1782838800')
+    })
     it('should offset date literal with timezone 00:00 specified', function () {
       return test('{{ "1990-12-31T23:00:00+00:00" | date: "%Y-%m-%dT%H:%M:%S"}}', '1990-12-31T17:00:00', undefined, opts)
     })