real time clock on MDBT42Q

Like

20 (parts) per Hundred  (parts) = 20 per 100 = 20% = 0.20

so

20 parts per Million (parts) = 20 per 1,000,000 = 0.000020. 

Since an average month has 30 x 24 x 60 x 60 = 2,592,000 seconds, about 2,600,000 seconds = 2.6 M seconds , 20ppm means 20x2.6 = 52 seconds +- accuracy per month, about 1 Minute per Month or roughly 2 Seconds per Day.