[4.2] Database : ensure equivalent dates are stored

So that inputing a date with a different timezone than the application timezone, will not store the same date (as the timezone is not store), but at least an equivalent date.
Fix https://github.com/laravel/framework/pull/5882
This should be documented. Maybe add `However, we must ensure the incoming DateTime is in the same timezone as the application before it is later converted to a string (losing timezone information) or the represented time will be silently shifted.`