Duration breaks Rules (as provided by CCK module).

CCK fields are tested in content.rules.inc using the function array_diff_assoc. This function cannot deal with objects. So it tries to convert the duration into a string, which fails due to missing __toString() method.

As a workaround, the following patch implements the missing function.

In general. CCK and Duration should agree which is a useful design. I expect the current implementation to have some other traps breaking functionality.

As a side effect it fixes a warning generated, when strict error checking is used.