Calendar.naive_datetime_from_iso_days

You're seeing just the callback naive_datetime_from_iso_days, go back to Calendar module for more information.
Link to this callback

naive_datetime_from_iso_days(iso_days)

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Specs

naive_datetime_from_iso_days(iso_days()) ::
  {year(), month(), day(), hour(), minute(), second(), microsecond()}

Converts iso_days/0 to the Calendar's datetime format.