archive_calendar caches too aggresively

archive_calendar caches based on the $start_of_month:

$cache = cache_get("archive:calendar:$start_of_month");

this means that the css classes 'day-selected' and 'day-today' are only set once per cache period. the result is an archive calendar block that hilights the wrong day as 'today', etc.

i propose either ... a) eliminating the cache altogether (this block can always be throttled if that's an issue) or b) at least caching on a per-day view.

i'd be happy to supply a patch for either approach.