Am I right with my assumption that ISO8601 does not allow zero differences to UTC as a timezone indicator (for GMT in this case), but requires to use the Z indicator?
I want to get a second opinion on the interpretation of
The zone designator is empty if use is made of local time of the day in accordance with 5.3.1.1 through 5.3.1.3, it is the UTC designator [Z] if use is made of UTC in accordance with 5.3.3 and it is the difference-component if use is made of local time and the difference with UTC in accordance with 5.3.4.2.
because I fear one could argue that if I don't want to normalize the time but want to indicate that this is the local time used, you could still use 5.3.4.2.
So is 1985-04-12T10:15:30+00:00 a valid string according to ISO8601 or is only 1985-04-12T10:15:30Z valid?
After reading @Ole V.V. answer I saw that wikipedia refers to a newer version of ISO 8601. After reading a newer draft dated 2016 I found out:
4.2.5.1 Difference between local time and UTC of day When it is required to indicate the difference between local time and UTC of day, the representation of the difference can be expressed in hours and minutes, or hours only. It shall be expressed as positive (i.e. with the leading plus sign [+]) if the local time is ahead of or equal to UTC of day and as negative (i.e. with the leading minus sign [-]) if it is behind UTC of day.
(ISO/TC 154/WG 5 N0038 ISO/WD 8601-1 2016-02-16 )
So obviously booth 1985-04-12T10:15:30+00:00 and 1985-04-12T10:15:30Z are valid but 1985-04-12T10:15:30-00:00 is not.