timer.go has a timer struct for scheduling things to happen once every n
days before or after a given time. Pass a string list of day deltas to
parse, a unit to parse these as (only 24/-24 hours really make sense),
then call Check() on the returned struct with your "since" time, and the
time a timer was last fired. If one goes off, store the time so you can
pass it in subsequent calls. To be used in the user daemon for "remind
every N days" functionality. Was initially gonna allow more precision
than days, but ran into problems, most likely from me overcomplicating
it and not wanting to store too much data. some tests also in
timer_test.go.
