I have 2 custom fields in the opportunity like Start date and End date
The format for them is Start Date:-  10/20/2010 10.59PM
                               End Date :-    11/31/2010   10:59 PM
The time will be set automatically whatever the time the system shows.

Now, the requirement is that  Star Date : Any date  12:01AM and End date : Any Date 11:59 PM.

So the time should be displayed like that no matter what ever the created date and time and it is for every oppotunity.

Is it possible  to do write any formula field or trigger, please help me on this?