private void CalculateEndDateFromDuration()
{
DateTime date;
TimeSpan stime;
if(!DateFromInput(startday.Text, startmonth.Text, startyear.Text, out date)) return;
if(!TimeFromInput(starthour.Text, startminute.Text, out stime)) return;
date = date.Add(stime);
TimeSpan dur;
if(!GetDuration(out dur)) return;
date = date.Add(dur);
endday.Text = date.Day.ToString();
endmonth.Text = date.Month.ToString();
endyear.Text = date.Year.ToString();
endhour.Text = date.Hour.ToString();
endminute.Text = date.Minute.ToString("00");
}