If you want to add it for yourself, this is all you have to do:
1. Create a new page.
2. Click in the HTML source box.
3. Paste in the following code:
And that will automatically create a calendar for the current year, up to the current month.
Suppose you want the calendar to go back from 2011, or from any particular day. Just add parameters to the script, like so:
In that example, the calendar will start linking days on March 24, 2011.
* Note - the script will create links for days that don’t have any posts, but checking whether not a post exists on a certain day would take a ton of hits on tumblr’s servers, so the script will just link all days in the time range.