fix date encodings - #21

This is a copy of #15 with a cleaned up diff.

I do think it would be a bit safer to use the UTC string with milliseconds removed, rather than re-encoding the offset. Happy to do that if you agree.

I do think it would be a bit safer to use the UTC string with milliseconds removed, rather than re-encoding the offset

The timezone is significant information in git commits, if we drop this information, commit hashes will be changed and no longer match.

Sorry, I think I meant ISO string.

Right now the code lops off milliseconds and timezone data, then adds back the timezone data using date.offset...I think we could just slice out the milliseconds, and the ISO timezone string would match what the code currently calculates using date.offset.

As long as the tests pass and information isn't thrown away, I'm not particular on how it's implemented.

My understanding is JS Date objects can't store timezones. Make sure to test on several JS engines in case on non-standard behavior that makes it pass on just some.