how to find 'enddate' after adding some days to a 'startdate'

1) I would not use Textboxes instead use DateTimePickers for the date information.
2) Use a NumberUpDown for the number of days to increment by.

So you have 3 controls

DateTimePicker StartDate
DateTimePicker EndDate
NumberUpDown   NoOfDays

EndDate.Value = StartDate.Value.AddDays(NoOfDays.Value);

Something like that should do the job.

You could put some handling in the ValueChanged event of the numberupdown control so that when the value is altered EndDate is automatically updated.