Rules currently in force

On 2001-02-01, a decree was published in Mexico's Diario Oficial, modifying the country's time zone rules. The rules for the year 2001, to remain in force until modified by some future decree, are as shown in the table. The times of transition are local time as measured before the transition takes place, unless otherwise specified.

Distrito Federal wanted to exempt itself from DST. The Supreme Court denied the exemption in April, 2001, and the district did observe DST. Since then, the court has issued a new ruling. If nothing else changes, Distrito Federal is free to remain on standard time next summer (2002).

Time zone history

Sources: This information was adapted from the tz database. Zone names come from that database. The names were chosen according to certain rules, but don't necessarily match any names used in the "real world".

The information from the tz database has been slightly modified in light of a report  prepared by research assistants to the Mexican Chamber of Deputies. It summarizes the provisions of eighteen decrees that affected Mexican time standards from 1921 to 1999.

Example: When it was 14:18 (or 2:18 p.m.) local time on 1985-04-20 (April 20, 1985) in Ensenada, Mexico, what was the UTC?

Ensenada is in the state of Baja California Norte. From the first table, Baja California Norte is in the America/Tijuana time zone. (This is a specialized meaning of "time zone", which is more precise than common usage.) Its code letter is 'J'. In the second table, look in the column headed 'J', and in the row headed '1982-12-02 00:00' (which is the latest date not exceeding the one you're trying to convert). At the intersection of that row and column is "-8 R2". That means that, on the given date, Ensenada was in a time zone whose standard time was UTC-8, and was following Rule R2 to determine the period of daylight saving time. Looking in the third table, by Rule R2, April 20 was not in the daylight saving period, so no adjustment is needed. The offset at that time was UTC-8. Add eight hours to the given time to get UTC. The answer is 1985-04-20 22:18 UTC.

Time zone names

In Mexico, time zones are called husos horarios. Formally, each time zone is identified by its central meridian. For example, the zone where standard time is UTC minus seven hours is called la hora del meridiano 105° oeste de Greenwich. During the daylight saving period, this same area is said to observe la hora del meridiano 90°.... Daylight Saving Time is usually called Horario de Verano. Standard time is known as Horario de Invierno, or occasionally hora oficial or hora normal. Time zone names are rarely used in Mexico. When they are used, they're not consistent. The names in the Guía Roji, a Mexican road atlas, are simply translations of the corresponding U.S. zone names. The abbreviations in the tz database don't have any official standing, but they, too, are based on the U.S. time zones.

Source	UTC-6	UTC-7	UTC-8
Parliamentary Gazette 2000-03-29	Zona del Centro	Zona Pacífico	Zona Noroeste
Decree of 1923-12	Hora del Este	Hora del Centro	Hora del Oeste
Decree of 1930-11-15	Hora del Golfo H. del G.	Hora del Centro H. del C.	Hora del Oeste H. del O.
Guía Roji 1995	Hora Oficial del Centro	Hora Oficial de las Montañas	Hora Oficial del Pacífico
tz database	CST, CDT	MST, MDT	PST, PDT

The list of decrees  says (translated), "Before time zones were implemented in Mexico, standard time (also known as Hora del Ferrocarril - Railroad Time) was the time of the meridian of Tacubaya." Tacubaya was the site of an observatory in Mexico City, now functioning only as a point of interest. Its mean solar time would have been GMT minus 06:36:48. Railroad Time was eliminated by the decree of 1932-01-21.