Fixed point? What would that accomplish? Rounding creep happens, irregardless of data type, every time it rounds the last digit, fixed or float.
Fixed point is never a good idea, bad idea or not, it does speed up things on limited hardware. A missile isn't "budget" though.
Yes floats are difficult, every operation moves it farther from original guess, it's just guessing the last digit. Only solutions are to not do fractional math at all, or to reload and adjust values periodically. Time keeping however is a subject that been already well researched. Any embedded platform I've seen has at least a dozen app-notes and a dozen different ways to keep accurate time.