Date.to_iso8601

You're seeing just the function to_iso8601, go back to Date module for more information.
Link to this function

to_iso8601(date, format \\ :extended)

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Specs

to_iso8601(Calendar.date(), :extended | :basic) :: String.t()

Converts the given date to ISO 8601:2019.

By default, Date.to_iso8601/2 returns dates formatted in the "extended" format, for human readability. It also supports the "basic" format through passing the :basic option.

Only supports converting dates which are in the ISO calendar, or other calendars in which the days also start at midnight. Attempting to convert dates from other calendars will raise an ArgumentError.

Examples

iex> Date.to_iso8601(~D[2000-02-28])
"2000-02-28"

iex> Date.to_iso8601(~D[2000-02-28], :basic)
"20000228"

iex> Date.to_iso8601(~N[2000-02-28 00:00:00])
"2000-02-28"