If I'm going to use a 64 bit type for time I'd probably just use int64 microseconds, have over 250,000 years of uptime before overflowing, and not have to worry about the precision changing the longer the game is active.

So using fixed point. You could do that, but you can't make every single time-using variable fixed point without a lot of unnecessary work. Without sufficient care you end up with less precision than floating point. If you don't want to spend a ton of upfront time on carefully optimizing every variable just to avoid wasting 10 exponent bits, default to double.