They can straight modify the date without any dev tools or hacking. The field is no longer grey and you can click on it and choose a date. I just tested submitting a date and it does not come through in the variables.

I am using Cloud at the moment which does not have a build number.

I can recreate the issue as well. As you stated, it seems to just be a UI issue in that the field appears to be editable. However, if you did change the date to a different value and submitted, it won't actually save, so the submission data is still correct.

We'll report this to our development team for further investigation. This has been filed as bug 499142.

Regards

Here is a copy of my test process

I am using Cloud at the moment which does not have a build number.

For Cloud, stating the date you reproduced the issue (and/or when you first noticed it occurring) is equivalent to providing the version number to us as we can check internally which Cloud build was in Production at that time.