Having recently acquired a DSO, I was curious to see how accurate the timebase was. The built-in frequency counter tied to the triggering seemed decent enough - although not enough digits to really get a solid determination. While fiddling with a long time scale on a fast signal (was looking to get a high resolution FFT at the time), I went well past the Nyquist limit, and realized I could use that to compare timebases of my signal source and scope. Below is a screenshot taken with a 10MHz sine wave from my signal generator into the scope (used two channels out from the signal generator into channels one and two of the scope, just to convince myself there was no timebase variation between the channels). Note that the sample rate is 2.5MSa/s - so we are aliasing big time. The frequency of the aliased signal gives the delta from the the scope's timebase compared to the signal generator's. By adjusting the signal generator up or down that amount, it was trivial to determine which direction it was off - in this case the scope was on the low side. I had previously done an analysis of my signal generator compared to a WWV disciplined clock and determined it was low by 0.4 ppm. The scope is an additional 1.6 ppm lower than the signal generator (16 Hz / 10MHz) for an absolute error of -2ppm. Of course, if I had a solid reference like a Rubidium source, or GPS disciplined receiver, I could have gotten to the errors even faster, with more confidence.
One side note, there is some amount of wiggle on the aliased signal - you can see a std dev of about 0.5 Hz - so the relative jitter between the two devices is not too bad. Can't tell which one is contributing more to the jitter.
Just wanted to share a fun use of aliasing to quickly gauge how accurate (or more correctly how closely matched) two timebases are. All in all, I'm pretty pleased with the absolute accuracy of both units - both well within spec.