you can use the Compare method of date time to check the date.
for example:
DateTime date1 = Convert.ToDateTime(txtbox1.Text);
DateTime dt = DateTime.Now;
int result = DateTime.Compare(date1, dt );
string relationship;
if (result < 0)
relationship = "is earlier than";
else if (result == 0)
relationship = "is the same time as";
else
relationship = "is later than";