(core) Set DateTime timezone during xlsx import

Summary:
DateTime columns had a blank timezone after xlsx imports because the
timezone was not included in the column type. We now append the
document's timezone to the type of all imported DateTime columns.

Test Plan: Server test.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3896

app/common/RelativeDates.ts

@@ -98,6 +98,14 @@ export function formatRelBounds(periods: IPeriod[]): string {
   );
 }
 
+/**
+ * Returns a new timestamp that is the UTC equivalent of the original local `timestamp`, offset
+ * according to the delta between`timezone` and UTC.
+ */
+export function localTimestampToUTC(timestamp: number, timezone: string): number {
+  return moment.unix(timestamp).utc().tz(timezone, true).unix();
+}
+
 function formatDay(quantity: number, refUnit: IPeriod['unit']): string {
 
   if (refUnit === 'week') {