The hidden until setting is being lost when a grade category is edited.

Steps to reproduce:

1. In a course with gradable items go to the Grade administration page (Click the grades link the course administration block) and click the setup tab. A tree of grade items and grade categories should be shown.
2. For the course level grade item at the top of the list, expand the "edit" menu and click "Edit settings". This will take you to a grade category edit page.
3. In the the "Category total" group, click the "show more..." link to display advanced controls.
4. Check the "Hidden" checkbox
5. Set a date other than today's date in the "Hidden until" setting and check the "Enable" checkbox
6. Click Save changes. You should be returned to the Categories and Items page.
7. For the course level grade item, expand the "edit" menu and click "Edit settings" again. You will be returned to the grade category edit page.
8. In the the "Category total" group, click the "show more..." link again. The "Hidden" setting will still be checked however the "Hidden until" setting will not be enabled and will show the current date where it should be enabled and show the date you set.

It appears that the date is actually being set when the form is originally saved, but is not being loaded into the form when it is reloaded and lost if it is not re-entered.