Your birth is an event that happened at an instant in time, but very few systems concern themselves with that detail. The vast majority need to store birth _date_ and have no interest in the time or location.
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.
