I often need to check what the month and date is during my work day. (I can remember the year :) This calendar is very simple - a holder and three rings, one for the months (1-12), one for the day 'tens' (nothing, 1, 2 or 3) and one for the day 'ones' (0-9). The rings fit in the holder from the back, and today's date appears in the window. I printed the month in a different color (orange) from the day (purple). You can have the US order month-day or the European order day-month. You change the date manually by repositioning the rings.
I printed it on a Makerbot 2 in PLA using standard settings, no supports. The wheels could probably benefit from being printed at higher resolution to get the numbers to look cleaner.