Others have addressed the federal government's legal right to set standards of weights and measures, but I just want to add that there's a legitimate interstate commerce argument as well. Prior to the application of time zones, train schedules were nearly impossible to keep consistent as every town had it's own 'time zone' being aligned to their solar noon.

Time is already crazy for programmers and logistics, but imagine it being essentially a continuously changing band with longitude with slight local variations. The modern economy relies on a sane, more or less consistent view of time.