Date Time Picker Validation

In a form, the user is required to choose date and time in a picker, thereafter choose other fields and hit 'Done'.

I have a datetime picker that displays a date and time by default. Moreover, the user is allowed to type and erase the date and time. (by using keys and backspace). In case a user doesn't fill an entire date as an entry. I have to choose between either of the following two options:

  1. Whether the date time picker should auto-fill the recent value on the user's behalf?

  2. Or should the input field stay half-filled and then throw a 'Validation Error' like "Please Choose a Date and Time" ?