Almost all of the other replies are excellent. In light of your question I think you'll find interesting this realtime map that shows how far out of whack the regional grids are from 60Hz: http://fnetpublic.utk.edu/frequencymap.html
Going forward it may not be a good idea to rely on the grid's timebase as there's talk as well as an NPRM out there to do away with the standard. https://www.federalregister.gov/documents/2010/03/29/2010-64...
All power grids are connected, so there is just one frequency.
http://www.nerc.com/AboutNERC/keyplayers/PublishingImages/NE...