> In the nVersion bits proposal that I co-authored we solved that issue by
> comparing the timestamp against the median time, which is guaranteed by
> the protocol rules to monotonically advance.
>

I'm also a fan of using the median time to ensure that there is a clear
point where the protocol change starts. Something like "blocks only allow
the larger block size if the associate pindex has pindex->GetMedianTimePast()
after midnight 11 Jan 2016 and where a supermajority showing support for
the fork has previously been reached".