During a research I was doing, I considered various periods of 13.

From 21st December to 2012 I counted 13 months. November 2011, the mayan crystal skull ceremony (with 13 skulls). I also considered the last 13 years, back to 1999, the eclipse on august that year was predicted by the mayans and held some importance..

I also counted the last 13 centuries..
And the last 13.000 years.

13.000 years ago was 10.988 B.C.

Thanks for the thread.