how to measure the time error of the arduino?

[shahardgn]

hi,
i need to know the time error on the arduino in two ways
one, i need to know if i start a stopper watch for aprox. 0.5s, what will be the error
second, if Im collaborating several arduino's and whaiting a month, what will be the time difference between them?
thank you

[robtillaart]

1) Check my stopwatch class - https://github.com/RobTillaart/Arduino/blob/master/libraries/StopWatch/

2) Depends on Crystal versus resonator.

If you want to keep them in sync you should use the time library and sync every minute/hour/day with an external source e.g. an RTC or by means of NTP.

[westfw]

You have said "measure" in several messages.  do you MEAN measure, or do you mean "predict, based on the specifications in the datasheets"?