The timeRangeSelectingStartEndFormat property (string) specifies the date/time format to be used by the target position indicators. The indicators can be enabled using timeRangeSelectingStartEndEnabled property.
The string uses format described in DayPilot.Date.toString() method documentation.
The "Auto" value resolves to "H:mm" (24-hour clock locale) or "h:mm tt" (12-hour clock locale).
"Auto"
dp.timeRangeSelectingStartEndFormat = "h:mm tt";