It was started today. The oldest comment on the thread [1] was 3 hours a go.

Also, TheOatmeal just posted on his Facebook page: “$100,000 in 3 hours and 20 minutes” [2]

[1] http://theoatmeal.com/blog/tesla_museum

[2] http://www.facebook.com/theoatmeal

You could look at one of @oatmeal's tweets about how long its been in existence and work backwards to figure out the start time...