DVDdoug:
If that's not good enough, I assume there are ways of "trimming" the oscillation frequency (with capacitors?). I'm pretty sure my watch (and RTC modules) are better than 50 ppm, so either they are buying super-good crystals or they are trimming.
The DS3231 has both a trim register, and temperature compensation. It beats 50ppm by a mile. One of these days I plan to actually trim one to GPS time but, a trim value of zero is always so close as to make it seem OCD to change it.