By default, the Scheduler works with exact points in time. E.g. a one day event starting on 2015-10-01T00:00:00 ends on 2015-10-02T00:00:00 (duration of 24 hours).
However, you can adjust how the Scheduler treats the end date to "Date-only" using .eventEndSpec property:
dp.eventEndSpec = "Date"; // default value is "DateTime"
See also:
http://api.daypilot.org/daypilot-scheduler-eventendspec/