OS X doesn't offer a very good menu bar clock by default. Day-O is better.
By Matthew Malinowski, 2013-10-24
First of several "new Mac OS X install" posts to remind me of my settings the next time I do this.
OS X's default menu bar clock is not very flexible or useful. Shaun Inman created a simple, much better alternative called Day-O. It works perfectly with Mavericks.
Option-drag the system clock out of the menu bar to remove it. Download Day-O, copy it to Applications, open it up. Click on the new clock in the menu bar and go to Preferences, and configure away.
The first half of my clock is an ISO week date, which looks like this (as of today):
2013-W43-5. In other words, it's Thursday of week 43 of year 2013. The string for this is
The second half of my clock is the date and time, which looks like this (as of right now):
2013-10-24T11:42:17-0400. Clear enough what that is. The string for this is
So, the whole string together is
yyyy-'W'ww-c yyyy-MM-dd'T'HH:mm:ssZ, which displays