I assume you are talking about the month control:
DayPilot works with full DateTime specification for both the event start and end. This means the end date is read as an exact time point, i.e. "2015-09-14" means "2015-09-14T00:00:00".
In the Pro version, there are options to read the end date as Date only in selected places:
.allDayEnd property of the Calendar ("DateTime" or "Date"; "DateTime" is the default value)
.eventEndSpec property of the Scheduler ("DateTime" or "Date"; "DateTime" is the default value)