A calculation of a time interval across the leap second has a similar ambiguity. For example, it can be difficult to establish the time-ordering of events in the vicinity of a leap second, since the time 23:59:59.2 in the leap second occurred after 23:59:59.5 in the first second with that name. The time tag corresponding to23:59:59 is therefore ambiguous, since two consecutive seconds have that name. That is, they repeat the binary time equivalent of 23:59:59 twice, and the next second is second 0 of the following day. The next best thing is to add the extra leap second by stopping the clock for one second at 23:59:59, and that is what the NIST time servers do. The name of a positive leap second is 23:59:60, but systems that represent the current time as the number of seconds that have elapsed since some origin (NTP, for example) generally cannot represent that time.
local time in Colorado (Mountain Daylight Saving Time, UTC-6). For example, a leap second at the end of June will occur June 30 at 5:59:59 p.m. The leap second event is linked to the UTC time scale (not local time as with daylight saving time), and therefore occurs at different local times in different time zones. The event can be scheduled for any month, but the months of June and December are preferred, and no other months have ever been used.
The leap second is added to the last minute of the last day of a month. The following discussion describes only the insertion of a positive leap second for this reason. The Internet Time Service and Leap SecondsĪ leap second is announced in advanced in Bulletin C of the International Earth Rotation and Reference Service ( The leap second can be either positive or negative, although only positive leap seconds have ever been used, and it is very unlikely that negative leap seconds will ever be required. For details about the UT1 server, please see the UT1 NTP Information page. NIST is now offering a network time service to deliver UT1 time. There are some steps you may have to take when accessing the NIST Internet Time Service through a firewall. Here are the server names, locations, and IP addresses and current status. NIST operates several stratum-1 network time servers, which means their time is directly linked to UTC(NIST), the official NIST time. In the settings, you may be allowed to pick which NTP server the time comes from.
Mac: Applications > System Preferences > Date & Time Windows: Double-click the system clock and then click on the Internet Time tab. Windows, Mac, Linux) have an option to automatically synchronize the system clock periodically using an NTP (network time protocol) server:
#Mac utc clock code#
We are phasing out the ftp access to the time servers.Īll of the public files on the time servers, including example source code for programmers who want to write their own applications, have been moved to this public ftp site: If you wish to subscribe to this list, please send your name and email address to: internet-time-service (subject: Subscribe%20to%20ITS%20mailing%20list) NIST has established a mailing list (Google Group) to inform users of status changes of the Internet Time Service. Set your computer clock via the Internet using tools built into the operating system Two-Way Satellite Time and Frequency Transfer (TWSTFT).
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