NISTIME_FAQ.LIS - Info and FAQ file for NISTIME

DESCRIPTION OF THE AUTOMATED COMPUTER TIME SERVICE (ACTS)

The service uses multi-speed modems which should automatically adapt to
the speed of the originating modem.  All messages are sent using standard
ASCII characters with 8 bits, no parity and 1 stop bit.

Format of the time message:

                                     D  L D
              MJD  YR MO DA H  M  S  ST S UT1 msADV        <OTM>
             47999 90-04-18 21:39:17 50 0 +.1 045.0 UTC(NIST) *
             47999 90-04-18 21:39:18 50 0 +.1 045.0 UTC(NIST) *
             etc...

     The message transmits Universal Coordinated Time (UTC), the
official world time referred to the zero meridian of longitude.

The MJD is the modified Julian Day number, which advances by 1 at 0000 
UTC every day.

The DST parameter provides information about Daylight Saving Time, using 
the model valid for the continental US:

  00 = US is on standard time (ST).    50 = US is on DST.
  99 to 51 = Now on ST, go to DST when your local time is 2:00 am and the
             count is 51.  The count is decremented daily at 00 (UTC).
  49 to 01 = Now on DST, go to ST when your local time is 2:00 am and the
             count is 01.  The count is decremented daily at 00 (UTC).

LS = Leap second flag is set to "1" to indicate that a leap second is to be
added as 23:59:60 UTC on the last day of the current month (usually June or
December).  The flag will be set to "2" if the last second of the current 
month is to be dropped. The second following 23:59:58 UTC on the last day 
of the month will be 00:00:00 of the next day in that case.  The flag will
remain on for the entire month before a leap-second event; it will be 0 
otherwise.

DUT1 = Approximate difference between earth rotation time (UT1) and UTC, in
steps of 0.1 second.         DUT1 = UT1 - UTC

The specified time is valid when the "*" on-time marker is received.  This
character will be transmitted 45 ms early to compensate for the nominal
delay in the modems and the telephone connection.

[Note: ACTS will compute the actual rond trip delay if the program sends back
the * character as soon as it is received.  If ACTS receives four consecutive
echos that fall within a certain range of variation, it will calculate the
round trip time and divide by two.  It uses this value to compute more
accurately when it should send the time marker.  ACTS then sends a # character
instead of a * to indicate that this synchronization has been achieved.]

The maximum connection time will be 40 seconds unless you transmit a 
"%" character before then.  If this character is received, the transmitter 
will break the connection at the next on-time marker.

For more information write:
NIST-ACTS
Time and Frequency Division
Mail Stop 847
325 Broadway
Boulder, CO  80303

e-mail: time@time.nist.gov

Software for setting (PC)DOS compatible machines is available
on a 360-kbyte diskette for $42 from:
Office of Standard Reference Materials
NIST, Gaithersburg, MD, 20899, (301) 975-6776

The software and additional information are also available 
on the internet via anonymous ftp from time_a.timefreq.bldrdoc.gov.

From the internet:
                    Automated Computer Time Service (ACTS)
  __________________________________________________________________________
                                       
NIST's Automated Computer Time Service (ACTS) has been provided since 1988 to
those users who need to synchonize computer clocks to the correct time. The
telephone number for ACTS is (303) 494-4774. Using ACTS requires only a
computer, a modem, and some simple software. When a computer connects to ACTS
by telephone, it receives an ASCII time code. The information in the time code
is then used to set the computer's clock.

  [INLINE]
  
ACTS works at speeds up to 9600 baud with 8 data bits, 1 stop bit, and no
parity. To receive the full time code, you must connect at a speed of at least
1200 baud. The full time code is transmitted every second and contains more
information than the 300 baud time code, which is transmitted every 2 seconds.
The full time code looks like this:

JJJJJ YRMODA HH:MM:SS TT L DUT1 msADV UTC(NIST) OTM

where:

JJJJJ is the Modified Julian Date (MJD). The MJD is the last five digits of the
Julian Date, which is the number of days since January 1, 4713 B.C. To get the
Julian Date, add 2.4 million to the MJD.

YRMODA is the date. It shows the last two digits of the year, the month, and
the current day of month.

HH:MM:SS is the time in hours, minutes, and seconds. The time is always sent as
Coordinated Universal Time (UTC). An offset needs to be applied to UTC to
obtain local time. For example, Mountain Time in the U. S. is 7 hours behind
UTC during Standard Time, and 6 hours behind UTC during Daylight Saving Time.

TT is a two digit code (00 to 99) that indicates whether the United States is
on Standard Time (ST) or Daylight Saving Time (DST). It also indicates when ST
or DST is approaching. This code is set to 00 when ST is in effect, or to 50
when DST is in effect. During the month in which the time change actually
occurs, this number deincrements every day until the change occurs. For
example, during the month of October, the U.S. changes from DST to ST. On
October 1, the number changes from 50 to the actual number of days until the
time change. It will deincrement by 1 every day, and reach 0 the day the change
occurs.

L is a one-digit code that indicates whether a leap second will be added or
subtracted at midnight on the last day of the current month. If the code is 0,
no leap second will occur this month. If the code is 1, a positive leap second
will be added at the end of the month. This means that the last minute of the
month will contain 61 seconds instead of 60. If the code is 2, a second will be
deleted on the last day of the month. Leap seconds occur at a rate of about one
per year. They are used to correct for irregularity in the earth's rotation.

DUT1 is a correction factor for converting UTC to an older form of universal
time. It is always a number ranging from -0.8 to +0.8 seconds. This number is
added to UTC to obtain UT1.

msADV is a five-digit code that displays the number of milliseconds that NIST
advances the time code. It is originally set to 45.0 milliseconds. If you
return the on-time marker (OTM) three consecutive times, it will change to
reflect the actual one way line delay.

The label UTC(NIST) indicates that you are receiving Coordinated Universal Time
(UTC) from the National Institute of Standards and Technology (NIST).

OTM (on-time marker) is an asterisk (*). The time values sent by the time code
refer to the arrival time of the OTM. In other words, if the time code says it
is 12:45:45, this means it is 12:45:45 when the OTM arrives.

Since the OTM is delayed as it travels from NIST to your computer, ACTS sends
it out 45 milliseconds early. This always removes some of the delay. Better
results are possible if the user's software returns the OTM to ACTS after it is
received. Each time the OTM is returned, ACTS measures the amount of time it
took for the OTM to go from ACTS to the user and back to ACTS. This quantity
(the round-trip path delay) is divided by 2 to get the one-way path delay. ACTS
then advances the OTM by the one-way path delay and the OTM changes from an
asterisk to a pound sign (#). When the # sign appears, the time code is
synchronized within a few milliseconds of UTC(NIST).

ACTS is a very popular source of time. The service currently has 12 phone lines
and receives about 10,000 telephone calls per day.

  __________________________________________________________________________
                                       
Click here (53k) to download WINACTS, a simple ACTS program for Windows.

Click here to download PC-TIME, a simple ACTS program for MS-DOS.

For questions or more information about the Automated Computer Time Service,
contact:

Michael Lombardi, email: (lombardi@bldrdoc.gov).
  __________________________________________________________________________
                                       
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