On Tue, Jul 4, 2017 at 6:30 PM, shaolinfry via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Some people have criticized BIP9's blocktime based thresholds arguing they
> are confusing (the first retarget after threshold). It is also vulnerable
> to miners fiddling with timestamps in a way that could prevent or delay
> activation - for example by only advancing the block timestamp by 1 second
> you would never meet the threshold (although this would come a the penalty
> of hiking the difficulty dramatically).
>
> On the other hand, the exact date of a height based thresholds is hard to
> predict a long time in advance due to difficulty fluctuations. However,
> there is certainty at a given block height and it's easy to monitor.
>

You could get most of the best of both with a combination of the two: Have
the activation be a timestamp plus a certain number of blocks to come after
maybe about 100, which is more than enough to make sure all the games which
can be played with timestamps have passed but a small enough amount that it
doesn't add much uncertainty to wall clock time.