Currently, Duration exposes a bunch of canned fields like Zero, OneMinute, 
NegativeOneTick, and so on.  It also exposes sensible operator overloads and 
factory methods, meaning that it's unclear as to whether 'a duration of three 
days' should be best constructed via:

Duration.FromStandardDays(3)
or
Duration.OneStandardDay * 3

I suggest that we pick one, and I'd tend toward the factory methods, rather 
than trying to provide a bunch of fields for 'common but not really' durations.

Alternatively, we should remove the factory methods (apart from FromTimeSpan()) 
and rationalise the common durations (removing NegativeOneTick in favour of 
-OneTick and adding OneStandardWeek).  But I'd prefer the former, I think.