leap seconds; [0-60] is ok

Bob Devine ncifcrf!decwrl.dec.com!devine%cookie.DEC
Thu Jun 2 18:34:27 UTC 1988

> When the first leap seconds were added, which I believe were in 1973,
> there were two inserted at the same time.  Thus a single legal value has
> had the seconds denoted as "61".  From memory, these two leap seconds
> were:
> 	1973 June 30 23:59:60
> 	1973 June 30 23:59:61

  No, only one second is ever added at a time.  However, Ron is right
about the first year (1972) containing two leap seconds.  Below is
the relevant section describing how leap seconds are inserted/deleted.

Bob Devine
[ On a sidenote to time, the Herstmonceux Castle where the Greenwich
Observatory is located is for sale.  You can buy it for only ~$15M.]

  From the NBS Special Publication #432 (issued in 1979):

    When leap second adjustments are necessary to keep the
    broadcast time signals (UTC) within +/- 0.9 second of
    the earth-related UT1 time scale, the addition or deletion
    of exactly 1 second occurs at the end of the UTC month.
    By international agreement, first preference is given to
    December 31 or June 30, second preference to March 31 or
    September 30, and third preference to any other month.

    When a positive leap second is required -- that is, when
    UT1 is slow relative to UTC -- an additional second is
    inserted beginning 23h 59m 60s of the last day of the
    month and ending at 0h 0m 0s of the first day of the
    following month.  In this case the last minute of the 
    month in which there is a leap second contains 61 seconds.

    Assuming that unexpected large changes do not occur in the
    earth's rotation rate in the future, it is likely that 
    positive leap seconds will continue to be needed about
    once per year.  If, however, the earth should speed up
    significantly at some future time, so that UT1 runs at a
    _faster_ rate than UTC, then provision is also made for
    negative leap seconds in the UTC time scale.  In this
    case, exactly one second would be deleted at the end of
    some UTC month, and the last minute would contain only
    59 seconds.

    Positive leap seconds were inserted in all NBS broadcasts
    at the end of June 30, 1972 and December 31, 1972 thru 1978.

    [ All years since 1978 have had leap seconds.  Bob]

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