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Annex F
(informative)
Time scales and epochs
F.1 General
TAI (Temps Atomique International) time base maintained by the Bureau International des
Poids et Mesures is the international standard for time based on the second. TAI is
implemented by a suite of atomic clocks and forms the timekeeping basis for other time scales
in common use. Of these, UTC is the time scale of most engineering and commercial interest.
The UTC representation is specified in ISO 8601 as YYYY-MM-DD for the date and hh:mm:ss
for the time in each day.
The rate at which UTC time advances is identical to the rate of TAI. UTC time differs from the
TAI time by a constant offset. This offset is modified on occasion by adding or subtracting
leap seconds.
Starting on 1 January 1972 – which is MJD (Modified Julian Day)
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41 317 –, the world’s
standard time systems began the implementation of leap seconds to allow only integral second
correction between UTC seconds (offset TAI) and conventional civil UTC time expressed in
days, hours, minutes and seconds. Leap second corrections are made preferably following
second 23:59:59 of the last day of June or December. The first such correction, a single positive
leap second correction, was made following 23:59:59 on 30 June 1972.
The MMS UTC epoch began at 0 hours on 1 January 1970 (MJD 40 587). Times measured
are designated in this standard as MMS UTC seconds. MMS UTC Time is represented in
terms of seconds and fraction of seconds. The seconds representation would overflow in 136
years or roughly January of the year 2106.
The MMS Btime6 (TimeOfDay) type shall be an OCTET STRING. A value of the TimeOfDay
type may contain either 4 or 6 octets. The first form specifies the time as the number of
milliseconds since midnight on the current date (the date is not contained in the value), while
the second form contains the time and a date, expressed as the relative day since 1 January
1984. The first four octets shall contain a value indicating the number of milliseconds since
midnight for the current date in both forms.
F.2 Standard time sources
There are two standard time sources of particular interest in implementing systems for which
MMS UTC time is required by the application.
The first are systems implementing the SNTP protocol widely used in synchronizing computer
systems within a network. The SNTP-servers themselves are synchronized to timeservers
traceable to international standards. UTC time accuracy from SNTP systems is usually in the
millisecond range. SNTP uses the same time format as NTP. While SNTP provides time
synchronization within one network, NTP allows a global time synchronization on the internet.
SNTP provides the current time, the current number of leap seconds, and the warning flags
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The Julian Date, JD, is the Julian Day Number, JDN, followed by the fraction of the day elapsed since the
preceding noon (1200 UTC). The Julian Day Number is a day count with the origin, JD = 0, at 1200 UTC on 1
January 4713 b.c. The Modified Julian Date, MJD, is the Julian Date less 2 400 000,5 which shifts the origin to
midnight on 17 November 1858. For example: at 0 hours on 1 January 1900, JD = 2 415 020,5;
JDN = 2 415 020, 16:57:44 and MJD = 15 020.