DateTime.opBinary

[bachmeier via Digitalmars-d-learn]

I was just reading through the documentation for std.datetime. 
DateTime.opBinary looks pretty interesting:

http://dlang.org/phobos/std_datetime.html#.DateTime.opBinary

Does anyone know how to use it? You certainly can't learn 
anything from the documentation, because duration is a mystery. 
If someone knows, I can submit a PR with that information added.