When is midnight? Nomenclature question (was Re: Guatemala DST?)

Markus Kuhn Markus.Kuhn at cl.cam.ac.uk
Sun Apr 23 09:04:03 UTC 2006

"Dave Cantor" wrote on 2006-04-22 22:34 UTC:
> On 22 Apr 2006 at 16:06, Oscar van Vlijmen wrote:
> > DST [for Guatemala] starts on Sunday, April 30, 2006 at 12:00 Midnight local
> > standard time DST ends on Sunday, October 1, 2006 at 12:00
> > Midnight local daylight time
> I am not commenting on the veracity of the cited material at all,
> and I quote Oscar only because the quotation serves as an 
> example.
> I'm not even sure that people on this list are the right ones to 
> complain to, but surely most everyone on this list, must have 
> noticed the ambiguity.
> I am troubled by the specification of midnight on a certain date.
> In the old days (and by that I mean roughly before computers were 
> commonly used by non-computer-geeks like us) to keep time, 
> "midnight Tuesday" meant the minute after 11:59 p.m. Tuesday 
> night.  I think (but am not sure) that most people still mean 
> that when they say "midnight Tuesday".  Midnight _used_to_be_ a 
> synonym for 2400 hrs., the end of the day.
> But we pretty much don't use 2400 hrs. any more, and "midnight" 
> has become a synonym, in some contexts, for 0000 hrs., the start 
> of the day.  So "midnight Tuesday" might refer to the minute 
> before 12:01 a.m. Tuesday morning (Monday night!).
> Is anyone else concerned?  What should be done?  Who should do it?

Government officials who use obsolete and ambiguous terms such as "12:00
midnight" should be gently pointed to the relevant official standards
for time notation, which have solved this problem adequately and

ISO 8601: 00:00 is midnight at the start of the given date
          24:00 is midnight at the end of the given date
                (= 00:00 of the next day)

All this is discussed in great detail, for example, at


Markus Kuhn, Computer Laboratory, University of Cambridge
http://www.cl.cam.ac.uk/~mgk25/ || CB3 0FD, Great Britain

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