These docs are for an old release.
Info on upgrading to v5
Event::setStart
Sets an event’s start date.
event.setStart( date, [ options ] )
date is something that must parse into a Date Object.
options is an optional plain object that can have the following property:
maintainDuration— if set totrue, the event’sendwill also be adjusted in order to keep the same duration
If you’d like to set both the start and end at the same time, use setDates instead.
If there are other events that share the same groupId, those events’ dates will be similarly modified.