Calendar.naive_datetime_to_string

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

naive_datetime_to_string(year, month, day, hour, minute, second, microsecond)

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Specs

naive_datetime_to_string(
  year(),
  month(),
  day(),
  hour(),
  minute(),
  second(),
  microsecond()
) :: String.t()

Converts the datetime (without time zone) into a string according to the calendar.