(core) Fix one day shift in datepicker for DateTime column [WIP]

Summary:
Datepicker reads date using getUTCDate() but we used to parse date
using the document timezone instead of utc, hence the 1day shift.

Test Plan: Manual testing.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3801

app/client/widgets/DateEditor.js

@@ -72,7 +72,8 @@ function DateEditor(options) {
         toValue: (date, format, language) => {
           const timestampSec = parseDate(date, {
             dateFormat: this.safeFormat,
-            timezone: this.timezone,
+            // datepicker reads date in utc (ie: using date.getUTCDate()).
+            timezone: 'UTC',
           });
           return (timestampSec === null) ? null : new Date(timestampSec * 1000);
         },