[FIX] calendar edit item timezone

Solves a fullcalendar issue when trying to add an event in a particular day - fullcalendar works in the timezone defined by the display timezone preference. It submits back to the server the timestamp of the chosen day at 12:00am UTC. Server used to think this timestamp is 12:00am in user's timezone which made the day go backward/forward by 1 day in some timezone setups. The fix consist of correctly formatting beginning of the chosen day at the user's timezone in the backend.