How do I check to see if Variable....Date

I am learning C# and need some help with my code:
I have a code below and need to validate that the variable has a datevalue, how do I accomplish this task?
Goal: When a DateTime is passed through my Static method, how do I check for a DateTime value.  If DateTime InputValue is NULL I need to return nothing, if the string has a DateTime Data I need to return that DateTime value -- Your help is greatly needed and apprieciated thank you.
Code (C#)
public static DateTime IsNullDateTime(System.Object InputValue)    <-- also should I have System.Object or DateTime - please advise?         { ...