how to change input timing to 1920x1080@60Hz

I recently changed to dual Dell U2311H monitors. One shows the message:

The current input timing is not supported by the monitor display. Please change your input timing to 1920x1080@60Hz or any other monitor listed timing as per the monitor specifications.

Last time this message came up, it also went away after a day or two. But then it came back up, right when I really needed both monitors (of course!). When I go to Displays, I don't see any numbers like 1920x1080.

How do I change it?

My Mac Mini is running OS X 10.8.4

My thanks,

Karen