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

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),