1998 Chevy Tahoe Distributor turned 17 degrees from 0

[jclick]

I have a "check engine" code that Pep Boys could read because it was a GM Ignition code they couldn't decipher. I took the truck to GM and they determined the code meant that the distributor has been turned 17 degrees. I guess the guy that owned the truck before me turned it for some reason. Anyways, what's involved in getting this back to zero? Is there any type of mark showing where zero is?

Thanks for the help.