how to measure the time error of the arduino?

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

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.

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