It's the ideal solution (and one I already implemented with a Keygen-Date header), yes, but this bug was discovered months after the feature was introduced and

It's the ideal solution (and one I already implemented with a Keygen-Date header), yes, but this bug was discovered months after the feature was introduced and relied upon (I run an API product). So it's not as simple as using a different header, since previous clients will still be on the Date header. Backwards compatibility can be a bitch.