Can you set the default time on a date/time field

If this is on a form, then you can have a formula calculate that date / time value and then have a form rule to populate the date/time field if the date time field is blank.

The formula would be something like

var  TimeOfDay TOD = ToTimeOfDay("8:00 am");

var date TwoDays = today() + days(2);

ToTimeStamp($TwoDays, $TOD)

What if I want the default for a date field to be something other than today.  Don't want a rule because that means I have to interact with the form.