Tray Calendar is a simple software that places a date in the system tray.

(From Author) Tray Calendar is a freeware utility that I wrote after being amazed that
1. Microsoft did not bother to offer us a calender in the tray, and
2. People were charging good money for tiny little tray apps.

The idea is simple: Put the day of the month next to the clock in the system tray. It tells you todays date, and will update the number displayed when the date changes. 20 seconds after it starts it will reshuffle itself so that it is the right-most icon in the tray.
This is useful if you put it in the "Startup" folder (so it starts automatically) becuase during startup other icons such as virus checkers and such may get between the calender and the clock.
The only options are that you can disable date checking, disable auto-shuffle, and you can change the auto-shuffle time. Right click on the date icon for the options dialog.

The license of this software is Free, you can free download and free use this calendar software.