Possible BC break with timezone handling

Looks like with 6687602b73064d2e8ce6708b7018b2dd02bf2beb / 6d0ad766ca525c7a3aee921b66554ddf21a5c023 / 46498224624f419d915ce1a14afb399dc4d95850, we did two things which break BC: removed `date_tz` and `modified_tz`, and began stripping timezones from date strings.

While I agree these should be changed, they're a BC break in 1.x, so we need to revert at least the `*_tz` part.

I *think* we can get away with stripping timezone information, but we should check that.