DEFAULT_DATE_FORMAT
DEFAULT_DATE_FORMAT = 'Y-m-d\TH:i:sP'
Default date/time format of backend is 'ISO 8601'
Note, last letter is 'P' and not 'O':
O - Difference to Greenwich time (GMT) in hours (e.g. +0200) P - Difference to Greenwich time (GMT) with colon between hours and minutes (e.g. +02:00)
Backend uses the 'P' variant