The term I learned is SWAG -- Scientific Wild-A*** Guess.
Without reading through all of your text, I'd say you're having a problem subtracting 7-digit numbers with 6-digit floating-point precision. There's a reason the Unix time() function returns an integer time in seconds rather than floating point.
NASA's Orion Flight Software Production Systems Manager Darrel G. Raines joins Planet Analog Editor Steve Taranovich and Embedded.com Editor Max Maxfield to talk about embedded flight software used on the Mars on EE Times Radio. Live radio show and live chat. Get your questions ready.