Mac OS X only: If you don't want to install a whole other piece of software to see today's date in your menubar, you can add it manually in System Preferences. It's not an obvious checkbox or dropdown, so bear with me. After the jump, the step by step.
- In System Preferences, open International and go to the Formats tab.
- Next to Dates, click the Customize button. Arrange your date in the format you want to appear on your menubar. Use the down arrows on each element to choose between formats (January, Jan, 01, 1, etc.) I chose [Oct 29], as shown.
- Now, select the entire date format and copy it to your clipboard (Cmd+A, Cmd+C). Hit OK.
- Hit the Customize button next to Times. Choose the "Medium" format.
- Paste the date format on your clipboard next to the time format, and hit OK. You're done!
Note that the day of the week (like "Mon") will appear next to your date and time by default. To turn that off, in System Preferences, go to Date and Time, and uncheck "Show the day of the week." Thanks, Snowmoon!
