On 13 December 2016 at 19:12, Paul Eggert <eggert@cs.ucla.edu> wrote:
Perhaps something like this?

Officially, in the 1918 and 1919 fallback transitions the clock for the long day kept going after 24:00, reaching 24:59 the previous day before falling back an hour to 00:00 the next day. As this behavior is not representable in this database, it is modeled instead as the clock reaching 00:59 the next day before falling back to 00:00.

I think something like that is best.  Certainly the kind of peculiarity one can appreciate here!  =)

Looking at those decrees (again with only Google Translate to help me) though, I don't think they actually called the time 24:59 or 25:00, but rather just 01:00.  From the 1918 one Carlos linked (emphasis added):

S. M. el Rey (q. D. g.) se ha servido disponer que para el cumplimiento del articulo 2. del Real decreto de 3 de Abril último, relativo al cambio de hora, la duración legal del día 6 de Octubre próximo será de veinticinco horas, al término de las cuales, y cuando los relojes marquen la una, se retrasarán hasta las veinticuatro, para comenzar las cero horas del día 7.

HM the King (D.D.G.) has served to provide that for the fulfillment of Article 2 of the Royal Decree of April 3, relative to the change of time, the legal duration of the next October 6 will be twenty-five Hours, at the end of which, and when the clocks mark one, they will be delayed until twenty-four, to begin the zero hours of day 7.

So the legal length of 1918-10-06 was 25 hours, and included the hour after midnight, which we here would call 24:00:00 to 24:59:59, before the new day began.  Likewise in 1919.  The legal distinction just raises the interesting point about how we would represent that timestamp in modernity.

As an aside, this got me thinking about possible alternative ways of implementing DST where the (legal) days are all of equal length.  Indeed, if we just numbered each day in DST from 01:00:00 through 24:59:59 rather than 00:00:00 through 23:59:59, that sure would be an interesting way of handling it!  (Alas, that's not what happened here.)

--
Tim Parenti