How is calldate generated in freepbx?

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The cdr_adaptive_odbc.conf file contains the line "alias start => calldate"
The ${CDR(start)} variable returns a time that differs from what is entered in the CDR table by a second, there is a suspicion that the result may differ by 2-3 seconds or match calldate. How can I get the exact time that will eventually be written to the database?