Natural time zones based on longitude, a proposal for addition to the TZ Database. This includes longitude-based time zone definitions, data files and lookups, allowing devices to opt-out of daylight saving time and still interoperate with those who follow it
by Ian Kluft December 1, 2025 (updated December 8, 2025)
The Longitude Time Zones project will be undergoing a major update.
The narrow overlay time zones intended to be hyper-local in scope were originally defined as 1 degree of longitude wide, making each 4 minutes of clock time. While 1 degree seemed like a mathematically clean definition, it didn’t match anything people have in real life. It has been difficult to try to advocate for that part of the proposal. That was what caused a re-think of the interval on the narrow overlay time zones.
Actual time zones in effect since 1986 which are not at 1-hour boundaries, all have 30 or 45 minute offsets from the hour. For example, 30-minute time zone offsets are used in South Australia, India, Newfoundland and others. 45-minute time zone offsets are used in Central Australia and Nepal.
Consideration was also given to 5-minute intervals since that would cover other recent history back to 1972. But 15-minute intervals are less to ask of the public, and therefore more likely to be able to successfully advocate for a standard.
With the narrow time zones no longer set at 1 degree intervals of longitude, their names can’t use “Lon” as a prefix. For example, Lon000E would have been equivalent to UTC. Or fill in the longitude from 180 E to 180 W.
In the interest of simplicity, the new prefix for the narrow time zones is the same “East” or “West” as the hour-based time zones, with 2 additional digits for the minutes of offset: 00, 15, 30 or 45. With 96 in total going from 48 east “East1200” to UTC equivalent “East0000” to 48 west “West1200”.
Many changes will be needed: