Calendar.truncate

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

truncate(microsecond_tuple, atom)

View Source (since 1.6.0)

Specs

truncate(microsecond(), :microsecond | :millisecond | :second) :: microsecond()

Returns a microsecond tuple truncated to a given precision (:microsecond, :millisecond or :second).