I am using 20 PPM external crystal oscillator in RTC because of 20 PPM it shell be give 1.33 sec delay per day how can I calculate the error in this external crystal.
There are 86,400 seconds in one day so the error is 1.33 in 86,400. Work out the % error and apply it to the oscillator. Without knowing the frequencies involved it's difficult to give more advice. An error as small as that can normally be controlled by adjusting capacitors around the crystal.