What more than:
d1 = DateTime.Now;
d2 = DateTime.Now.AddDays(12);
if (d2 > d1)
{
}
else
{
}
Do you want ?
Do you want to specify minimum and/or maximum intervals (TimeSpan) between the two Dates ?
Do you want a very specific DateTime validator where the criteria are fixed, or do you want a general purpose validator you can pass parameters into that handle a variety of conditions ?
How about excluding certain Date intervals if they include any weekend day, or any specific range of hours of day, or night ?
Let's see some of your code.