The visibleStart() method returns the start of the first visible day.
DayPilot.Date object
var dp = new DayPilot.Calendar("dp"); // ... dp.startDate = "2019-01-03"; // Thursday dp.viewType = "Week"; dp.weekStart = 0; // Sunday dp.init(); var start = dp.visibleStart(); // Sunday, 2018-12-30T00:00:00