Hello Viktor,
Interesting question thanks !
After having double checked you are totally right, so I will update the previous post.
We did not take into account the daylight saving time hour change that occurred the 26th of March 2017.
So 2017-03-19T09:00:00+01:00 is indeed 9:00 Paris time and 8:00 UTC
And 2017-03-26T09:00:00+01:00 is then 10:00 Paris time and 8:00 UTC
Using a good converter can be useful.
About RFC 3339, it is a profile of the ISO 8601 standard. They are basically the same concepts (except small subtle differences) but RFC 3339 needs more requirements to represent completely the date and time.
Here is a interesting thread about the subtleties between ISO 8601 and RFC 3339.