[FIX] calendar: don't read start_date(time) in read

It seems to randomly save a bad value into the cache for star_date(time) field.

For already impacted database, it can be easily fixed with a simple update.
  update calendar_event set start_datetime=start, stop_datetime=stop where allday = 'f';
  update calendar_event set start_date=start, stop_date=stop where allday = 't';

Than can cause errors for google synchro.