PLC scans periodically, the scanning time is tens of ms, the shorter the time, the greater the error. Why don't you change the timing to a pulse of 60S and compare it? The deviation of internal time after calibration is a problem of internal clock, which is related to the quality of PLC. As for not 7200 seconds, it has something to do with the time timer you read 1 second. 100ms resolution timer, timer bit and current value are refreshed when the instruction is executed. So its error should have two situations. The first case is that the scanning period is less than 100ms, and the error of the timer should be between 0-0-100 ms. In the second case, the scanning period is greater than 100ms, and the error of the timer should be the scanning period of the program.