This question stuck in my head. ClockMon may be just what you need. It was written for situations like the one you're descibing wherein Windoze loses time when it's busy, but the BIOS RTC (Real Time Clock) can be considered to remain fairly accurate. This is probably exactly what you were looking for - let me know if it works for you.
Here are some other interesting links, depending upon what you were looking to compare your WP's clock against (which you never really specified other than not having it dial the net on a regular basis).
The NIST ACTS is a non-net based dial up time service if you don't mind dialing the 303 (Colorado) or 808 (Hawaii) area codes. Of course, if you were willing to do this, then you would be willing to dial the internet, wouldn't you? I only pointed this out because I thought it was interesting to still have such a service in this day and age of "everything's connected" devices. So...
If your WP goes into suspend mode at all, then this MS support doc may be appropriate. You should also read this thread that discusses the WP's APM mode problems and a driver that san472 was kind enough to write to fix the problems with suspend mode.