Fix tests broken by subunit regex tweak
Date-advancing functions: relative to start OR end

Only matters for variable-length intervals (i.e., months).  Intended to
pave the way for month-based recurring actions.
Add monthly-recurring actions
Test monthly recurs: vis starts at BOD, due at EOD
Negative dates count from the end of the month
Monthly recurs with specified time-of-day
Check that paying rent late won't skip next month