fix a problem with maxDatetime badly initialized when TZ is set

[shtouff]

Hi,

I encountered a minor problem: when using timezone that is not GMT/UTC, either set to config or via the browser, the "to" datepicker is initialized with a max value set to UTC, which makes impossible for me to choose a time close to now().

I don't know JS, so maybe my patch isn't so good, but you got the idea.

Hope this helps,

rémi

[chorsley]

This sounds similar to #317 too. Potentially merge the two?