From what I understood, in theory, the format without the seconds is invalid because the value should respect the partial-time format from the RFC 3339. However, it seems that the browsers accept a value without the time-second part and don't display the "seconds" part this case.

We could support it too but do we want to? 🤔 That would certainly be a new behavior so a new feature.

Curently i solved it with a DataTransformer DateTimeNoSecondsTransformer.php

$builder->get('date')->addModelTransformer(new DateTimeNoSecondsTransformer());

Would be more convenient to have this integrated in symfony forms though.

The with_seconds option defaults to false. It is honored when the form doesn't have data (and when the browser doesn't display the seconds). But it's not honored when the form has data.

Since it's "working" when the form doesn't have data, I guess it could actually be treated as a bug. Especially since DateTimeToHtml5LocalDateTimeTransformer::reverseTransform() already handle strings without seconds. But I'm pretty sure there will be complaints. Our own tests rely on the current behavior.

Friendly ping to form experts @xabbuh @yceruto 😄 WDYT we should do?

Probably this is something nobody ever thought about. IIRC. the handling in reverseTransform() was added as some browsers didn't not always send the seconds. However not displaying seconds when with_seconds is false would be a BC break as that option is false by default).