Accessibility
DateTimeInput is one named group with a read-only text trigger named by the label; required sets aria-required and an invalid field sets aria-invalid and aria-errormessage on the trigger. The calendar button uses calendarLabel. The popover uses popoverTitle, which is not painted.
The popover holds a calendar beside a time list; the list uses timeListLabel. Reset, Clear and Apply use resetLabel, clearLabel when set, and applyLabel. Apply is the only primary action; Reset and Clear are secondary, not destructive.
Confirm footer. Day and time presses only change a draft until Apply commits. Reset restores the value held when the popover opened and stays open. Clear, when clearLabel is set, empties the draft and stays open. Escape and an outside press close without committing.
The trigger shows no zone name or offset. If the zone is part of the task, put it in the label or description.