One advantage of using a fake clock implementation is that tests are easy to 
write because the time doesn't change.  One disadvantage is that it's not 
possible to then test operations that rely on time advancing (e.g. retrying 
operations).

One option is to use a mock clock, but programming the expectations is tedious. 
A simpler solution is to use an automatically advancing fake, such that each 
read advances time by a specified amount.

It seems like it would be useful for StubClock to provide an AutoAdvanceBy 
optional Duration (or Nullable<Duration>?) property that could be set to a 
value to opt-in to this behaviour.