Well, I'm trying to fix a form where it has a field that you place in your time. Below it will be several feilds that when numbers are plugged into them they will decrease the orignal number you typed in.
And then plug in 3 hrs into field b along with 5 hrs into c.
a b c
  
And it automatically subtracts from field after I plug in the value.
So, basically, I want the value that they plugged in changed to fit that scheme; however, upon clicking back on the feild they can still see the number they entered in. I believe its possible, I'm just not exactly sure on how to do it all.
:"The only dumb question, is the question not asked!"
Well, if one has a time sheet into which a person enters start and stop times the time worked for a single day can be computed and then if one has the daily data for the pay period, one could also sum the time worked for each day and have the total time worked during the pay period and with some additional work could determine the amount and type of U.S. overtime. And this is one example of computing time intervals and totaling time values.
Your requirement can not be done, once one leaves a field it that field gets recalculated you will lose the previous value.