[sneak]

int64 representing unix epoch millis (in UTC) is what I usually use. You need to jump through an additional hoop to store timezone or offset.

You can, of course, do the thing that JS requires you always do and put an ISO8601 date in a string. This has the benefit of storing the offset data in the same field/var.

Javascript needs int64s as strings, I believe, because the JS int type maxes out at 53 bits.