Set default for custom field

Set default for custom field

We have a custom datetime field in our cases, and every time we create a new case, the due date field is empty and the time drop-down is set to 1am. I would like to set the default for same day, and time for end of current day by default, but allow this to be changed as needed.

I have looked at the automation section, but there doesn't seem to be any way of updating a date/time field there. Is this something that might be do-able as a custom function with scripting? If so, does anyone have any tips or suggestions on how to get started?