is there a handheld calculator on the market that lets you input a date and time,add or subtract a set amount of hours (eg 96 hours) and gives you the answer in the same format i.e date and time.
If you know how you can use Microsoft Excel for such calculations and a number of hand-held devices have Windows on these days.
I've never seen any calculators that can do this though subtracting 96 hours is like subtracting 4 days but I guess you would be doing more complex calculations than this.