Here are specified the rules on dates format.
There are two type of dates: absolute and relative.
And absolute date refers to a specific moment in time, independently by the timezone.
A relative date refers to a day (or an hour) that is the same across all timezones
(for example the 10:00 in Rome and the 10:00 in New York).
The dates are absolute unless specified otherwise.
The absolute dates have the format '2011-12-03T10:15:30+01:00[Europe/Paris]'.
It's the ISO-8601 with an optional part between the square parenthesis (with the timezone name).
In Java 8 the formatter is https://docs.oracle.com/javase/8/docs/api/java/time/format/DateTimeFormatter.html#ISO_ZONED_DATE_TIME.
The relative dates have the format '2011-12-03 10:15:30' (ISO-8601 without the timezone part).