Hallo Arif,

if you have a look into datasheet of PCF8583, tolerance of chip is specified to be +-5ppm on average. If you add the +-20ppm max deviation of a good quality 32.768kHz quartz (AEL) and temperature deviation of -0.04ppm/°C, then you get 5ppm + 20ppm + 0.04ppm/°C x 20°C = 26ppm.

And if you use a methode of calibration, either mechanically or by software, then even much less deviation is possible. Datasheet of PCF8583 tells, that overall average deviation of +-5 minutes per year can be achieved. +-5 minutes per year is 10ppm, or 0.9sec per day.

Here the according datasheets:

http://www.aelcrystals.co.uk/pdf-files...lPagef.pdf
http://www.amresearch.com/datasheets/pcf8583.pdf

For the DS1307 I haven't found a specification about average deviation of time base...

Kai