Having worked on an integration recently, I was just point out to a mismatch when using datetime between e.g. PC/Android and iOS.
The dates I am collecting are “YYYY-MM-DD HH:MM:SS” where iOS prefers to have it ISO style with ‘T’, being “YYYY-MM-DDTHH:MM:SS”
Although I just pushed the source format, I am not no longer sure what one SHOULD push to HA
From a general HA perspective, which date-format should one use to deliver integrations?
Generally speaking, HA doesn’t use the “T” / “ISO” version.
E.g., an input datetime returns its state as “2021-12-04 08:45:00”
OK and thanks
So, the issue I was faced with was with the flex-table-card where one uses ‘Date’ in calculations. With a non-ISO datetime this does not work on iOS and it is kind of a ugly hassle to workaround that…whereas with ISO date, on Android, PC and iOS it shows fine. The ISO format is as easy to use in templates with strp/strftime as long as one remembers the T 
From your experience, Is there anything ‘against’ using ISO?
My alternative was to offer both ISO and non-ISO date atributes but that too I find ugly
I think there was shift around the beginning of the year towards ISO and timezone. There was some commentary in github or something.
I only bumped into it because I was fiddling with an integration, and was struggling to find an example. So by no means an authority on the subject.