Reasons for a Leap Second Correction
A Leap Second is an intercalary1Intercalary: (of a day or a month) inserted in the calendar to harmonize it with the solar year, e.g., February 29 in leap years. one-second adjustment that keeps broadcast standards for time of day close to mean solar time. Leap Seconds are required to synchronize time standards with civil calendars, thus keeping UTC time in sync with the earth’s rotation.
Leap seconds can be introduced in UTC at the end of the months of December or June. The INTERNATIONAL EARTH ROTATION AND REFERENCE SYSTEMS SERVICE (IERS) publishes a bulletin every six months, either to announce a time step in UTC, or to confirm that there will be no time step at the next possible date. A Leap Second may be either added or removed, but in the past, the Leap Seconds have always been added because the earth’s rotation is slowing down.
Historically, Leap Seconds have been inserted about every 18 months. However, the Earth's rotation rate is unpredictable in the long term, so it is not possible to predict the need for them more than six months in advance.
Note: Leap Seconds only apply to the UTC and Local timescales. Leap Seconds do NOT affect the GPS and TAI timescales. However, a Leap Second event will change the GPS to UTC, and TAI to UTC time offsets. When a Leap Second occurs, NetClock will automatically change these offsets by the proper amount, no matter which timescale is currently being used by the system.
As of 2017 the GPS to UTC Offset is 18 seconds. The last Leap Second occurred on December 31, 2016.
NetClock can be alerted of impending Leap Seconds by any of the following methods:
- GNSS Receiver (if available as an input reference): The GNSS satellite system transmits information regarding a Leap Second adjustment at a specific Time and Date an arbitrary number of months in advance.
- Input references other than GNSS: Some of the other available input references (e.g., IRIG, ASCII, NTP) can also contain pending Leap Second notification in their data streams (see chapter below).
- Manual user input: NetClock can be manually configured with the date/time of the next pending Leap Second. On this date/time, the System Time will automatically correct for the Leap Second (unless the System Time’s timescale is configured as either GPS or TAI).
NetClock will announce a pending Leap Second adjustment by the following methods:
- ASCII Data Formats 2 and 7 (among other formats) from the ASCII Data option modules contain a Leap Second indicator. During the entire calendar month preceding a Leap Second adjustment, these Formats indicate that at the end of the current month a Leap Second Adjustment will be made by using the character ‘L’ rather than a ‘_ ‘ [space] in the data stream. Note that this does not indicate the direction of the adjustment as adding or removing seconds. These formats always assume that the Leap Second will be added, not removed.
- NTP Packets contain two Leap Indicator Bits. In the 24 hours preceding a Leap Second Adjustment, the Leap Indicator Bits (2 bits) which normally are 00b for sync are 01b (1) for Add a Leap Second and 10b (2) for Remove a Leap Second. The bit pattern 11b (3) indicates out of sync and in this condition NTP does NOT indicate Leap Seconds. The Sync state indicates Leap Seconds by indicating sync can be 00b, 01b, or 10b.
- PTP Packets provide leap indication with a 12-hour notification window.
- Some IRIG formats provide leap second notification indicators.
Note: It is the responsibility of the client software utilizing either the Data Formats or NTP time stamps to correct for a Leap Second occurrence. NetClock will make the correction at the right time. However, because computers and other systems may not utilize the time every second, the Leap Second correction may be delayed until the next scheduled interval, unless the software properly handles the advance notice of a pending Leap Second and applies the correction at the right time.
Leap Second Correction Sequence
The following is the time sequence pattern in seconds that NetClock will output at UTC midnight on the scheduled day (Note: This is NOT local time midnight; the local time at which the adjustment is made will depend on which Time Zone you are located in).
- Sequence of seconds output when adding a second ("positive Leap Second"):
- 56, 57, 58, 59, 60, 0, 1, 2, 3 …
- Sequence of seconds output when subtracting a second ("negative Leap Second"):
- 56, 57, 58, 0, 1, 2, 3, 4 …
Configuring a Leap Second
To manually correct the System Time for a leap second:
- Navigate to MANAGEMENT> OTHER: Time Management. The Time Management screen will be displayed. In the lower left-hand corner, the Leap Second Information panel will show if a leap second if pending. This panel will be empty, unless:
- A leap second is pending, and NetClock has obtained this information automatically from the GPS data stream.
- A leap second had been configured previously by a user via the Edit Leap Second window.
- To access the Edit Leap Second information window, click the GEAR icon in the Leap Second Information panel.
- The Edit Leap Second window will display:
- In the Leap Second Offset field enter the desired GPS Offset.
- In the Date and Time field, enter the date that the desired leap second should occur.
- Click Submit.
To delete a leap second correction, click the Delete button.
Note: The Delete button in the Edit Leap Second window will only be visible if a leap second has been set beforehand.