Your question is not quite clear to me, but if we have
DateTime? ExpireDateTime; // could be a variable or a property
it’s OK to say just
if (ExpireDateTime < DateTime.UtcNow)
{
...
}
This will be OK if ExpireDateTime is null (HasValue is false). Some inexperienced developers will struggle to understand lifted operators, so to make it more clear, you could write
if (ExpireDateTime < (DateTime?)DateTime.UtcNow)
{
...
}
It’s the same, but easier to read and understand.
Never write .Value if the nullable might be null, of course. You will get an InvalidOperationException “Nullable object must have a value” if you do so.