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Using a fromDate = 2022-07-14T23:03:10.000Z would include the registration record: fromDate = 2022-07-14T23:03:10.000Z minus 5 min. = 2022-07-14T22:58:10.000. The DateModified of the record is 2022-07-14 22:58:21.073 and therefore AFTER the fromDate. That is why it would include the record using a fromDate = 2022-07-14T23:03:10.000Z.
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Selecting the next fromDate
Calculating the fromDate for the next interval should be the date and time just before you issue the first GetCompletedRegistrationData call (or any other API endpoint with a fromDate). This ensures that you don't miss out any registration records due to the time the API calls and processing of the retrieved data takes.
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