Hi Mc and I,There are 3600 seconds in 1 hour, or 3,600,000 milliseconds in 1 hour.
Thus 1 ppm is equivalent to 3.6 milliseconds in 1 hour
20 ppm is 72 milliseconds in 1 hour.
Hi J,Many times the ppm specs for quartz crystals for oscillator control vary with their temperature and should be part of the spec. Maybe you have an incomplete spec. i.e, 20 ppm/degree Cent.
When the oscillator circuitry heats up, the crystal's frequency varies. To get a precision oscillator, the crystal will put into a tiny oven or a precision-isolated heat source. The heat keeps a constant temperature of the crystal's structure ensuring stable frequency.
See this link for more details https://www.everythingrf.com/community/tcxo-vs-ocxo