startOf is off when using timezones

[zaptree]

#1437

This fix does seem to fix some already failing unit tests as well. I'm not sure that it takes care of all the edge cases though especially the change in the utcOffset() which was added because utc.valueOf was returning the wrong value since it was using the (new Date()).getTimezoneOffset() since no $localOffset was set and Date.now() could have a different offset than the date we want because of DST