Thanks, Mikael Sundberg, for date refactoring

[Dave Briccetti]

Thanks, Mikael, for factoring the time-processing code out of
AgeCellRenderer into a new class. And for the unit tests to go with
it. Nice work.

Now, the rest of you, don’t wait for an invitation. :-) Jump in where
you can. Plenty to do in the Jira, or follow your own interests.