15 min intervals when setting time?
I don't think there's an easy way to achieve that without modifying the main lib files (not recommended) or creating a plugin.
The work around that I can think of at the moment is to create a separate dropdown list fields after the date field for time and minutes and set the minutes list options to 00, 15, 30, and 45.
I hope this helps you somehow.
PHP Programmer - interactivetools.com