Converts this date-time to a string in ISO-8601 extended format. For example, 2012-04-15T17:31:45.923452091-04:00[America/New_York] or 2020-02-13T02:30Z.
2012-04-15T17:31:45.923452091-04:00[America/New_York]
2020-02-13T02:30Z