Can't understand how to compare DateTime with opCmp

[Suliman via Digitalmars-d-learn]

I need to compare to DateTime. I looked at docs and found opCmp 
for DateTime type.
The problem is that I can't understand how to use it.
http://dlang.org/phobos/std_datetime.html#DateTime

opCmp(in DateTime rhs);

what is rhs?

I am trying to do something like this:
if( DateTime.opCmp(dtindb, outoftime));

But it's seems that I wrong understand how to use this function...