the datepicker extension is based on the jquery Datepicker
datepicker/
Ther is no time option for this extension
It is fairly straightforward to add a text or select list to your form in order to store the time as well.
If you don't mind doing some coding, take a look at
anytime/
if you want an integrated extension.
Personally, i never bothered, as a select list in addition to the date picker is sufficient for my needs