[LEAPSECS] Future time

[Warner Losh]

On Jan 20, 2014, at 10:27 AM, Tony Finch wrote:


> Warner Losh <imp at bsdimp.com> wrote:

>> On Jan 18, 2014, at 1:52 PM, Stephen Scott wrote:

> 

>>> The basis of my understanding is that UTC is a timescale that:

>>> -    progresses at a rate of the second (SI) and has done so since 1972-01-01.

>>> -    is expressed as a count in the form of date, hours, minutes and seconds;

>>> -    is continuous other than the discontinuities resulting from leap second corrections;

>> 

>> To pick a pedantic nit: It is continuous. No "other than". UTC is a

>> continuous time scale. It has a non-uniform radix that is expressed at

>> the leap seconds when its rules for labeling seconds changes slightly to

>> label the leap second 23:59:60. The timescale has a discontinuity when

>> there's a phase jump. UTC does not have phase jumps, even if it chooses

>> a non-traditional time to label some of its seconds once every year and

>> a half or two.

> 

> I think it is reasonable to say that UTC's labelling of seconds is

> discontinuous. For timescales that use simple ancient sexagesimal notation

> you can write down some straightforward formulae mapping between a count

> of seconds and a broken down time. If you try to do the same for UTC you

> have to write down something that is piecewise linear with exceptions. It

> isn't a continuous map like other timescales.


UTC defines second labeling such that it isn't technically discontinuous. It is an irregular radix that requires non-naieve math to properly convert to a count. UTC broke with thousands (or at the very least hundreds[*]) of years of time keeping practice by making what had been a completely regular radix notation for the time and converted to an irregular radix with known rules, but no known schedule.

Still a pain no matter what you call it.

Warner

[*] An interesting side note about days: The ancient Egyptians regarded light and night as two separate realms rather than as being two halves of the same day. The notion of the synodic day thus dates only from the new kingdom, somewhere around 1400BC where the two different realms, each governed by 12 starts was replaced by a system that unified them under 24 stars (http://www.scientificamerican.com/article/experts-time-division-days-hours-minutes/). It wasn't until the Greeks around 100BC that we see the hours becoming fixed length (although average people used seasonally varying hours for hundreds of years after this: not until mechanical clocks of the 14th century did this stop). From the 16th century until 1972, though, minutes and seconds were uniform set by the fixed hour. So although sexagesimal notation for time was invented thousands of years ago around the birth of Christ, it wasn't until really good mechanical clocks of the 16th century that one could say that the radix was truly fixed. Which leads me to why I equivocated.