We can examine this in different ways.
How many minutes are there in one year?
1 year = 365 x 24 x 60 = 525,600 minutes
Hence there are about 1 million minutes in 2 years.
If your clock was stable to 1 ppm, it might drift 1 minute in 2 years.
If your clock was off by 20 ppm, the error would be 20 minutes after 2 years.
Here is another view.
How many seconds in 12 days?
12 days = 12 x 24 x 60 x 60 = 1,036,800 seconds
Hence we can do the same analysis.
1 ppm = 1 second error in 12 days.
20 ppm = 20 second error in 12 days.
And another view.
There are 60 x 60 x 14 = 50,400 seconds in 14 hours.
An oscillator with 20 ppm error would have a 1-second error in 14 hours.
However, ±20 ppm does not tell the whole story.
You need to know about the temperature dependency of the crystal oscillator.
You also need to put a trimmer capacitor in your crystal oscillator circuit. Even though the crystal may have a ±20 ppm specification, the purpose of the trimmer capacitor is to reduce the absolute error to less than 1 ppm.
At 1ppm absolute error, an RTC might have an error of 1 minute in 2 years which is beyond most everyday time-keeping expectations.
I am not interested on online calculator !!!Maxim has a nice little calculator to assist in such matters
by Aaron Carman
by Jake Hertz
by Jake Hertz
by Aaron Carman