handling of time zones in admin interface doesn't seem correct

When editing a talk in admin mode, the times are converted between UTC and the timezone of the browser.

There are a few issues with that:

• there is noting making it clear that this is being done
• the conversion doens't happen correctly: just editing a talk without changing anything and saving it results in different times
• the TZ used is the TZ of the browser, not the TZ of the event

Maybe all times should use an explicit +XX offset.