Crystal oscillator error due to ppm

Question

In this link, the accepted answer states that there may be an error of 1 minute per month if the crystal ppm is 20ppm.

Can someone help me with the math of how the error of 1 minute is arrived?

Answer 1

PPM = parts per million. $$20 ppm = \frac{20}{1000000}$$

If you want to find how long it takes for the error to reach 60 seconds (1 minute,) then you divide the 60 seconds by the ppm value.

$$T_{seconds} = \frac{60}{\frac{20}{1000000}}= 3000000 seconds$$

Converting 3000000 seconds to days gives 34 days. That's a little over one month (31 days.)