The HeaderDateProperty determines the format of the date in row header in Timesheet mode (one day per row). It accepts the same values as DateTime.ToString(string) method.
JavaScript
The date format is determined by DayPilot.Locale.datePattern property. See also DayPilot.Scheduler.locale.
ASP.NET WebForms
The default value is "d" which will use the default format set for the current Culture.