A relative date puzzle

I'm looking for some ideas on how to handle the following get a datetime 
value that is relative to a specific future date when presented with a 
partial day &time value.

Specifically, I have an appl that requires some lengthy input involving 
days and times.  I have streamlined the d/e effort so that it can be 
done entirely using the number keypad (if available), or else just with 
numeric entries.  Works great, very quick (JS) and accurate.  Basically 
the users type some numbers and end up with something like:

Sat 08:00am  or Fri 07:00pm etc.

Does anyone have an idea on how I can convert this value to a true 
datetime value for my database updates, where the above value is 
relative to a specific date in the future?  IE, the d/e takes place a 
few days before a certain upcoming date and I want my entry of "Sat" to 
translate to the Saturday following that certain date.  This could take 
place one to two weeks prior to the start date, so just adding "next" to 
the value above won't work.  I really need to incorporate a specific 
date in the solution.

Thoughts anyone?