Fix the bug where the user cannot update start time and start date and sync it to Google Calendar
Fix the bug where the user cannot update the start time or date to a later time. This is because when the user inputs a start time or start date without an end time or end date, the system will set end time equal to start time and end date equal to start date. And if the user tries to push the start time or start date to a later time or date, there will be an exception thrown because the new start time or date is later than the end time or date.
Merge branch 'master' of https://github.com/cs2103aug2014-T13-2J/main