Not all properties are synchronized during the CallBack update - forcing the full update using DayPilotCalendar1.Update(CallBackUpdateType.Full) doesn't help.
1. You can place the Calendar control inside an UpdatePanel and refresh it traditionally (it will be created from scratch).
2. You can change the dayBeginsHour and dayEndsHour properties on the client-side using JavaScript and call .update():
dp.dayBeginsHour = 6;
dp.dayEndsHour = 20;
dp.update();
"dp" is the value of ClientObjectName property.
You can call this code in AfterRender handler (which is fired after CallBack updates):
http://doc.daypilot.org/calendar/afterrender/