Issues involved in computer time stamps and leap seconds (original) (raw)

The purpose of the leap second is to preserve the international definition of the calendar day duration as one rotation of the earth while allowing the duration of one second to be defined by cesium.

Three desirable characteristics for systems handling time

precise time (more so -- stable and precise frequency)

Elapsed time is based on an atomic frequency standard in a particular reference frame.
Every second that ticks is an SI second.
There are no "leap smears".
These are required for robust cell phones, GPS navigation, machine synchronization, financial market regulators, etc.

simplicity

Every "day" has 86400 "seconds" (60*60*24).
There are no lookup tables of when leap seconds happen.

calendar days

Days on the calendar are determined by the rotation of Earth.
This is the basis for the definition of Universal Time.

Of the above, a system can pick any two

precise time and calendar days

ITU-R TF.460 (originally CCIR recommendation 460)
CCIR chose this for radio broadcast time signals on 1970-02-03,
squelched astronomers who insisted that leap seconds would cause trouble,
and sold the leap second to the world as the perfect solution for all issues with time.
This choice made UTC the only time scale in which
the duration of one second and the duration of one day are defined and measured by different methods,
in which the clock is not related to the calendar.
Radio broadcast time signals implemented this on 1972-01-01.
During 1974-07 the 13th Plenary Assembly of CCIR named this time scale "Coordinated Universal Time (UTC)" in Rec. 460-1.
POSIX decided in 1988 that conforming systems MUST NOT use this time scale
(disregarding the existing implementation in the "right" time zone code).

calendar days and simplicity

Everyone had this understanding, that the clock is directly related to the calendar, since centuries before 1972.
POSIX (IEEE Std 1003.1-1988) chose this for computer time stamps
(disregarding advice about precise time, yet calling their computer time scale UTC).
Many other computer systems and languages chose this.
This is a Big Deal(TM) because contracts often specify that a vendor must supply a POSIX Conforming Implementation, but doing so means that a system trying to keep precise time cannot operate reliably during a leap second.

precise time and simplicity

Used in technical time scales (International Atomic Time (TAI), GPS system time, Galileo system time, BeiDou system time, Indian Regional Navigation Satellite System time, etc.) which are not concerned with calendar days.
Folks trying since 2001 to get ITU-R to abandon leap seconds in UTC have failed to achieve agreement for such a change.
Many governments have resisted a redefinition of calendar days as not being related to the rotation of Earth.
ITU-R WRC 2015 decided to retain leap seconds in UTC and defer action until 2023;
pundits believe this means ITU-R action is postponed until such time as external agencies forge agreement on the kind of time scale to be broadcast.

... somebody else's problem ...

The priorities chosen by each of the three groups above have been accompanied by lack of concern about the unhandled characteristic.

In 1970 when the CCIR decreed that leap seconds would start they had no algorithm for implementing them. It remains the case that the ITU-R has no responsibility for the ongoing implementation nor for any robust scheme of communicating the leap seconds to operational systems.

In 1988 when IEEE standardized POSIX they opined that computers do not keep precise time. Contrary to other cases where POSIX allows systems to implement a better scheme than required by the standard, in the case of leap seconds POSIX prohibits systems from doing the right thing.

The proposals to abandon leap seconds in UTC have not included text which describes the effect that would have on disconnecting the calendar from the rotation of the earth.

The documents from all of these groups have been silent about the issues that are not handled. Figuring out ways of handling the issues has been left as an exercise for other groups. The result has been that different systems have adopted different strategies.

There can be only one?

Reading through the history makes several things clear. In 1948, 1950,1952, and1954 astronomers proposed, resolved, and explained that a new kind of uniform time was needed for technical purposes, but that the old kind of calendar time was still needed for human purposes.In 1955 astronomers voted to redefine the second to have a duration that is unrelated to the duration of one day, but they voted in a context which explicitly understood that there was more than one time scale which could be used to assign labels to points in time. While voting to define the second in terms of a fixed duration (in a time scale of Ephemeris Time or Atomic Time), they also voted that the radio broadcast time signals should continue to provide calendar days based on observing the rotation of the earth (in the time scale of Mean Solar Time, a.k.a. Universal Time, as had been internationally adopted in 1884).

During the 1960s astronomers pointed out that precision timekeeping with atomic chronometers means that there must henceforth be two kinds of time scales. The physicists and radio scientists adopted the notion that it was impossibly confusing to have more than one time scale, and they ignored the objections of astronomers who said that a time scale with leaps would cause trouble for automated systems in continuous operation. The physicists, radio scientists, and bureaucrats decided that radio broadcast time signals should have seconds defined by observing cesium atoms and days defined by observing the rotation of the earth, and they led the CCIR to adopt leap seconds in UTC. During the 1970s they lobbied various international organizations and nations to adopt UTC with leap seconds as the perfect time scale for official and legal purposes.

The result of those actions during the 1960s and 1970s is this:
Instead of international agreement on two time scales (each of which has clear definitions and applications) we have international approval of only one time scale (UTC) for which almost no two agencies have agreed on what rules apply for its definition and purpose.

One time scale about which everyone disagrees has turned out to be far more confusing than two time scales each of which serves a clear purpose.

It is important to agree on what time it is

An underlying premise for all of the time-keeping technology that horologists have developed during the past centuries is that it is important to agree on what time it is.

One of the people who suggested the idea of leap seconds was Gernot Winkler of US Naval Observatory, but he also opined that leap seconds would be problematic for automated timing systems. After CCIR decided that there would be leap seconds, but before the change went into effect,Winkler decided that Navy time systems would not implement leap seconds

The saga of the leap second suggests that the parties involved believe that saying "No, we don't want to do it that way" is more important than agreeing on what time it is.

While viewing this page see also:

• One person who suggested the idea of leap seconds also pointed out that leap seconds would cause problems for automated systems. Before the first leap second happened he explained that USNO time systems would not use leap seconds.
• Three pictures with a short explanation of what leap seconds are about and why they cannot have a schedule.
• By law, how many seconds elapsed since 1972? The answer differs in various jurisdictions.
• In 1955 the duration of one second was redefined to be unrelated to the duration of one day
• a calendar of the ITU-R process of attempting to abandon leap seconds in UTC
• A history of precision time scales
• Plots and tables of length of day and ruminations thereon
• Plots of differences between time scales over long history
• Correct precision handling of leap seconds using code already on POSIX systems
• Plots of differences between time scales at year 2100

Steve Allen sla@ucolick.org

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