Real execution time differs from execution time installed by calculated timer per-schedule BR for twenty minutes and more

Dear Colleagues,

We've met unexpected behaviour of calculated timer BR execution where factual execution time differs from execution time defined in rule instance for hour and more. 

How could we fix this problem? Is there any way to monitor this situation? Any logs of other means to see what happens with timer and execution of per-schedule BR body?

Would appreciate very much.

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