On an iPad, choosing a To-do's When date, time or Deadline and tapping the picker's tick left the field open, and the value saved only after a tap somewhere else on the page. Leaving the page before that lost it.
A date chosen in its calendar now saves at once and closes the field. A time saves each time you turn its picker, and the field closes when you leave it. A date or time typed with a keyboard still saves on Enter or when you leave the field. A field still open when you go back or close the tab saves as the page closes. This applies to every date and time you change in place, not only on To-dos.