Automation-wise - it might be easiest to allow them to pick one date (say - the startdate), and an integer replresenting the day of the other "side", and set up your stored proc to calculate the second date, and then use that in whatever query requires it.
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?