RTC Accuracy?

I started to use my Cobra in a project, and noticed that the RTC was behind the actual time. I had set it off the NIST clock several months ago. I don’t recall the actual amount, but it was more than ten minutes slow in two and a half months . I am using a battery for the RTC.

I would think that the xtal used on the board is not extremely accurrate, and since it is not in an oven, it has a large drift potential.

Before I spend a lot of time characterizing the clock accuracy, I wonder if there is some expected accurracy for the real-time clock? Anyone have any experience with RTC accuracy?

I am making an assumption that the battery versus USB power is not an issue.