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To calculate the clock drift of your cRIO, you have to open the specifications document of your cRIO and to check the accuracy of the internal Clock. For example, the accuracy of the cRIO-9040's clock is the following :

The unit "ppm" means "parts per millions".

An accuracy of 40 ppm at 25°C means that for 1 million ticks of clock, the maximum deviation will be 40 ticks of clock.

Therefore, for one day the maxuimum deviation is : 86400 * 4e-5 = 3.46 secondes per day.

So in a month, the maximum deviation is : 3.46*30 = 103.8 secondes per month, or a little less than 2 minutes.