Calendar.time_to_day_fraction

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

time_to_day_fraction(hour, minute, second, microsecond)

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Specs

time_to_day_fraction(hour(), minute(), second(), microsecond()) ::
  day_fraction()

Converts the given time to the day_fraction/0 format.