Temp gauge rearview mirror is inaccurate

A few months ago I noticed that my temp gauge is not giving an accurrate read. Somedays it seems like it it pretty close other days it is 20 degrees off. Has anyone expereinced this?

2004 GMC Yukon SLT

Thanks

Justin

Did you calbrate it to your time zone? in the owners manual it shows you how to calibrate it.

I'm not trying to be sarcastic but what would the time zone have to do with the accuracy of my temperature gauge? Thanks in advance.