The startDate property sets the visible range start. In addition to DayPilot.Date object, it also accepts a string in ISO 8601 format ("2009-01-01T00:00:00").
Using DayPilot.Date object:
dps.startDate = DayPilot.Date.today().addDays(5);
Using date string:
dps.startDate = "2019-01-01";
Using date/time string:
dps.startDate = "2019-01-01T12:00:00";