An unknown number of NTP servers may mistakenly announce an upcoming leap second during October 2026. If this occurs some NTP clients’ time could briefly be in error by one second.

The atomic second used in coordinated universal time (UTC) is defined by a specified atomic transition in cesium (caesium) atoms is constant. The solar second, computed using the Sun’s position in the sky, has changed measurably over the past century. The leap second was introduced so that UTC would not drift appreciably from solar time. On dates announced in advance by the International Earth Rotation and Reference Systems Service (IERS), a leap second is added or subtracted from UTC. All previous leap seconds have occurred on June 30, or December 31. The most recent leap second occurred on December 31, 2016.

The Global Positioning System (GPS) distributes GPS-time, an indication of an upcoming leap second, and the GPS-UTC time difference (integral number of seconds). The specification for the second item, notification of an upcoming leap second, has sometimes been misinterpreted.

The Network Time Protocol (NTP), used to transfer time between two IP-connected computers, uses a two-bit leap second indicator to inform clients that “an impending leap second to be inserted or deleted in the last minute of the current month” [RFC5905].

On two separate days in November 2021, 256 weeks after the previous occurrence, a spurious leap second was advertised by several NTP servers. 256 weeks after that, in October 2026, some NTP servers are expected to again incorrectly advertise an upcoming leap second. One group of NTP servers may advertise on Saturday, October 24, 2026 and another group of servers on October 31, 2026. Some NTP servers contain logic to defer leap second advertisement until the last day of the current month, thus the October 31, 2026 date. A short technical discussion can be found here, the root cause is a GPS specification misinterpretation. The raw leap second data is provided correctly by the GPS receiver, but is incorrectly converted before being sent to the NTP server.

The November 2021 errors affected 150+ NTP servers, but I saw no reports of ill effects. The extent of any October 2026 errors is unknown, I’ve notified several administrators in advance. There are hundreds of millions NTP clients in the world. Whether any given client would be affected is an “it depends” question. One of the best ways to protect clients from spurious notifications is through the use of a leap seconds file, if that is supported in the client’s NTP software.

I will update this note with results later. It would be prudent to install the latest software from your NTP server vendor.

Photo by Duncan Sanchez on Unsplash.