DayPilotMonth get visible start and end date

It's not available directly but you can calculate it the following way:

On the server-side:

// assuming that StartDate is day 1 of the given month
DateTime startDate = DayPilot.Utils.FirstDayOfWeek(DayPilotMonth1.StartDate); 
DateTime endDate = DayPilot.Utils.FirstDayOfWeek(DayPilotMonth1.StartDate.AddMonths(1).AddDays(-1)).AddDays(6);

On the client side:

// "dpm" is the value of ClientObjectName
var startDate = dpm.startDate;
var endDate = DayPilot.Date.addDays(startDate, dpm.rows.length * 7 - 1);

I will make the results accessible using special properties/methods in the next release.