From a code standpoint, based off height is easy.

My first internal version triggered on block 406,800 (~May 5), and each
block increased by 20 bytes thereafter.

It was changed to time, because time was the standard used in years past
for other changes; MTP flag day is more stable than block height.

It is preferred to have a single flag trigger (height or time), rather than
the more complex trigger-on-time, increment-on-height, but any combination
of those will work.

Easy to change code back to height-based...



On Fri, Dec 18, 2015 at 2:52 PM, Jorge Timón <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> I agree that nHeight is the simplest option and is my preference.
> Another option is to use the median time from the previous block (thus you
> know whether or not the next block should start the miner confirmation or
> not). In fact, if we're going to use bip9  for 95% miner upgrade
> confirmation, it would be nice to always pick a difficulty retarget block
> (ie block.nHeight % DifficultyAdjustmentInterval == 0).
> Actually I would always have an initial height in bip9, for softforks too.
> I would also use the sign bit as the "hardfork bit" that gets activated
> for the next diff interval after 95% is reached and a hardfork becomes
> active (that way even SPV nodes will notice when a softfork  or hardfork
> happens and also be able to tell which one is it).
> I should update bip99 with all this. And if the 2 mb bump is
> uncontroversial, maybe I can add that to the timewarp fix and th recovery
> of the other 2 bits in block.nVersion (given that bip102 doesn't seem to
> follow bip99's recommendations and doesn't want to give 6 full months as
> the pre activation grace period).
> On Dec 18, 2015 8:17 PM, "Chun Wang via bitcoin-dev" <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> In many BIPs we have seen, include the latest BIP202, it is the block
>> time that determine the max block size. From from pool's point of
>> view, it cannot issue a job with a fixed ntime due to the existence of
>> ntime roll. It is hard to issue a job with the max block size unknown.
>> For developers, it is also easier to implement if max block size is a
>> function of block height instead of time. Block height is also much
>> more simple and elegant than time.
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