RB-7744: stop date selections jumping to the day before in timezones with a negative offset

The date selectors shifted the selected date(s) to a timestamp set at 0:00:00 UTC for each day. This is the correct value for sending to the engine but it caused visual problems in the pickers since these values would lose a day when converted back to the local timezone when that timezone had a negative offset.

To avoid this, the selectors work with the original local timestamps. Only when the user submits the timestamps are they processed. A second change is that the timestamps are sent to the engine in string format 1970/11/25 instead of as a timestamp 28335600, shifting the responsibility of converting to the "right" timestamp to the engine.