Avoid DateTimeZone

DateTimeZone can't represent all the valid timezones that we can handle
(including manual offsets). Unfortunately, it also doesn't support
subclassing DateTimeZone to support it.
Explicitly strip timezones from RFC3339 strings

This information is stored on the site, not for each post, so we can't
make any assumptions or assertions about what the timezone is. As per
RFC3339, the timezone part is optional, so we just strip it instead.
Check creating a post with irregular offset

Not strictly related to the rest, but double-checks timezone code.