What I find missing from most, if not all, date/time libraries is a three-arg timezone conversion function (i.e. one that is pure and doesn't rely on the current time or locale). Most timezone conversions take a zoned date and time and convert it to a different timezone. But it matters when/where you're doing the conversion for the reasons you're discussing. If you also supply a UTC timestamp indicating when the conversion should take place (note, not a zoned date and time, since the conversion to the universal timeline can be affected by DST changes), that conversion can be consistent no matter when/where the function runs.

I find this approach to be cleaner than saving wall time since it allows you to keep uniform times on the server and simplify server-side logic that acts on those times in bulk.