✨ Timezones - Add official support for UTC times

Pain

The date and time picker always shows and edits in the editor's local timezone, so a team cannot pin an event to the zone it actually happens in. Worse, a stored value carrying no zone is re-read as local time, so merely opening a document another editor saved shifts the stored instant by the offset between them.

The ui.utc escape hatch this issue was filed around no longer exists.

️ Approach

• Settle what official support means now: a per-field UTC boolean, or a general per-field timezone
• Implement it in the current picker
• Document it on the datetime field reference https://tina.io/docs/reference/types/datetime

✅ Acceptance Criteria

• A field set to a fixed zone shows the same time to every editor, whatever their machine is set to
• Opening a document and closing it without editing leaves the stored value unchanged

Screenshots

Related

Where the UTC option went: https://github.com/tinacms/tinacms/pull/5969 and https://github.com/tinacms/tinacms/pull/6174

From the original report: https://tina.io/docs/reference/toolkit/fields/date/#datetimepickerprops and https://tina.io/docs/reference/toolkit/fields/number/

cc: @isaaclombardssw, @wicksipedia, @JackDevAU