My birth certificate has location, day, year, hour, and minute of birth. Birth is an event in time, perfectly represented with a (UTC) timestamp.
Setting the bottom couple of bytes to zero achieves this, while maintaining nearly universal consistency with all other events in time that might need to be related with a packing of bits. People do it because it's what's being used at nearly every other level of the stack.
That's not the same thing as `time_t` though. `time_t` is UTC. UTC is not `time_t`.