how to check datetime has unassigned or default value?

If you have a DateTime object, then the default value is DateTime.MinValue, and hence you have to check it like this:

DateTime createdDate= new DateTime();

 if (createdDate==DateTime.MinValue)
 {
     //unassigned
 }

If the DateTime object is nullable, then here is a different story:

 DateTime? createdDate = null;

 if (!createdDate.HasValue)
 {
     //unassigned
 }

We can create extension method to check empty or unassigned datetime object as

public static class DateTimeUtililty
{
    public static bool IsEmpty(this DateTime dateTime)
    {
        return dateTime == DateTime.MinValue;
    }
}

And can be used as:

if (createdDate.IsEmpty())
{
       //unassigned
}