Turns out that after ac69394 invalid_message and invalid_message_parameters are no longer being propagated to datetime's children. What was the rationale for removing this, @bschussek?

This is really annoying...

The form framework itself also applies some validation. In this case it tries to transform the input to a DateTime object in a Transformer which fails. Such errors only get a generic "invalid" error message which cannot be customized. This is a known limitation.

Subscribing. I do have the same issue with a datetime field not validating whatever I do, with the same generic failure message.
@Tobion, how could I trace the failing DateTime transformation? I'm using symfony forms in version 2.3.6 on Silex.
Any known workarounds other than using a text or self-defined widget?

Edit: I've just found @bschussek's comment on Stackoverflow regarding this (http://stackoverflow.com/a/18367705/2373960) and I guess I've figured out where exactly my validation error stems from.

Using a custom form type is the only solution that comes to my mind.

Quick write up of what I did to get this working. Might not be the best solution, but it works.
In the end, I guess my issue was more a pebkac problem (i.e. me) than symfony forms.

The default widget with datetime is the 'choice' widget, which returns an array of values (year, month, day, ...) instead of a string. However, my widget returned a string, which made the Transformer fail with "Compound forms expect an array or NULL on submission." and thus the "This value is not valid" error message.

So what I did was to configure the datetime field to use single_text as its widget, with my custom widget overriding this to render a bootstrap enabled datetimepicker. Before that, I had overwritten the default datetimewidget, thus the choice widget, which of course wouldn't work.

Fixed in #9453.

You need to set the widget to single_text:
->add('created_on', 'datetime', array(
'input' => 'datetime',
'widget' => 'single_text'
))

@chaos-drone Please check if the issue persist with the latest 2.8 patch release (2.8.8 at this time) and open a new issue then.