Thanks for your feedback and fix...
... but... i'm a little bit confused.
No code change on geschnacke.de recently... i keep it on an older version to allow you testing. And it always should put out the ISO 8601 format like this: 2004-02-12T15:19:21+00:00
I'm not sure, how you could get a value without a timezone. Do you remember the time frame, when you saw this?
And btw.: It worked with #Pachli in the past until this specific version...
And next btw.: As mentioned: Newer versions of #Mammuthus will deliver a UTC time with Z notation via the more or less #Mastodon compatible client API.