Don't try to create the time string by hand, let the date formatters handle all the hard work.

Signed-off-by: Blake Skinner <bskinner@mit.edu>
General improvements to the date code. Get rid of as many constants as possible and let NSCalendar do the heavy lifting for us.

Signed-off-by: Blake Skinner <bskinner@mit.edu>
Use the (potentially) cached calendar instead of creating one from scratch.

Signed-off-by: Blake Skinner <bskinner@mit.edu>