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Automated Computer Time Service (ACTS)

clock and phone illustration

The Automated Computer Time Service (ACTS) has been provided since 1988 for users who need to synchronize computer clocks to the correct time. ACTS only works with analog modems that use ordinary telephone lines. Digital modems, such as Digital Subscriber Line (DSL), cable and wireless modems, cannot synchronize using ACTS. For computers with Internet access, the Internet Time Service should be used to synchronize to NIST.


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.

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 is the Modified Julian Date (MJD). The MJD has a starting point of midnight on November 17, 1858. You can obtain the MJD by subtracting exactly 2 400 000.5 days from the Julian Date, which is an integer day number obtained by counting days from the starting point of noon on 1 January 4713 B.C. (Julian Day zero).

YR-MO-DA 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. On the day of the transition from DST to ST, the code is set to 01. On the day of the transition from ST to DST, the code is set to 51. The client software is responsible for implementing the change at 2 a.m. on the day of the transition. Prior to the transition, the code is decremented as the transition day approaches. For example, November is the month of the transition (in the United States) from DST to ST. On November 1, the number changes from 50 to the actual number of days until the time change. It will decrement by 1 every day, and reach 01 on the day of the transition. It will be set to 00 the day after the transition, and will remain there until March 1, when the countdown for the ST to DST transition begins.

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 two 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).

The ACTS system in Colorado has four phone lines and receives an average of 700 calls per day. It can be reached by dialing (303) 494-4774.

The ACTS system in Hawaii has two phone lines and receives about 100 calls per day. It can be reached by dialing (808) 335-4721.


Created September 23, 2009, Updated May 2, 2023