These docs are for an old release.
Info on upgrading to v6
allDayMaintainDuration
Determines how an event’s duration should be mutated when it is dragged from a timed section to an all-day section and vice versa.
true or false (the default)
When true, the duration will remain roughly the same before and after it is dragged to/from an all-day section. “Roughly” because if an event has a duration with hourly precision, it will be rounded down to the nearest whole-day.
When false, the default, the event’s duration will be reset to defaultAllDayEventDuration (probably one day) if it is being dropped in an all-day section or defaultTimedEventDuration (probably one hour) if it is being dropped in a timed section.