DateTime[] start = new DateTime[] { new DateTime(1990, 1, 1), new DateTime(1995, 1, 1) };
DateTime[] end = new DateTime[] { new DateTime(2000, 1, 1), new DateTime(2008, 1, 1) };
TimeSpan timespan;
private void button1_Click(object sender, EventArgs e)
{
for (int i=0; i<2; i++)
{
if (end[i] < end[i + 1] && start[i] < start[i + 1] && start[i + 1] >= end[i])
timespan = (end[i] - start[i]) + (end[i + 1] - end[i]);
/* if (end[i-1] < end[i] && start[i-1] < start[i] && start[i] >= end[i-1])
timespan = (end[i-1] - start[i-1]) + (end[i] - end[i-1]) */
}
int timeDifference = timespan.Days;
MessageBox.Show(timeDifference.ToString());
}