They have only so many mantissa bits to spare and most of exponent bits and sign bit are unused.
Best practise is unsigned milliseconds. For 32 bits that would store every time point between 0 millisec and 49 days, 17 hours(edited)
Ay ! Well sometimes it's worth to re-structure, here probably not. Unless maybe there will come maps that heavly abuse adding to time ctr to simulate a unique score system. unlikely i guess