Modifying Default Time on a Date/Time field

I've created a custom date/time field for a form.  When this field is not populated and the user clicks on the calendar icon to populate the field, the time section defaults to midnight.  They can change it manually in 15 minute increments.  I also know the user can bypass the icon and simply type in the desired date/time.  My question is this.  When the user clicks on the icon is there a way to make the default time that displays in the fields below the calendar to be either a specific time like 5:00 PM or make it default to closest 15