Calculate a new date/time based on the input

Hello,

I store a time stamp in a variable called STIME.

For example, STIME = 7/26/27 10:05:51PM (always be some time after 7PM), I want to change the value to 7AM next morning.

That is, need to change STIME as 7/27/2007 07:00:00AM

How do I do this in a macro?

Thanks, Anil