hiding date field within multigroup gives "The dates are invalid" error

I have a date field and other fields within a multigroup (via cck 3.x).

The multigroup has fields like: date, user list, select lists, etc, so I am trying to hide parts of the group when the user logged in is different for what the user list is showing.

My code for hiding fields is based on: http://drupal.org/node/357328#comment-2259304

It works well with all the other fields, except the date field.

When I use it to remove a date field within a multigroup, it removes/hides the field. However, when a user now tries and saves changes I get "The dates are invalid" error on the date field. Not sure why, but it looks like it discards original data in the date field, when it should not be doing so

Hence this looks like a date field bug when in multigroup.

Please do you have any suggestions on how to get around this?

I also tried the read-only alternative code published in the thread http://drupal.org/node/357328. That
works only on text fields, and does not work on : date, user list, select lists...

Thanks for your help