Rules currently in force

Two new measures now being considered by the Mexican legislature (2009-12) would affect the country's time zones. One would harmonize the dates of time changes in northern Mexico with those in the adjacent parts of the U.S. This applies to points within 20 km. of the border, and would include Tijuana, Mexicali, Ciudad Juárez, Ojinaga, Ciudad Acuña, Piedras Negras, Anáhuac, Nuevo Laredo, Reynosa, and Matamoros. Ensenada would also be affected, although it lies much farther south. The other measure would move the municipality of Bahía de Banderas in Nayarit state from UTC-7 to UTC-6, because it is economically tied to Puerto Vallarta in Jalisco state.

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

Source: This information was adapted from the tz database. Zone and Rule names come from that database. The names don't necessarily match any names used in the "real world".

Note: I'm experimenting with an alternate presentation format for the time zone history.

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.) In the second table, look in the block of rows for America/Tijuana. Among them, look for the first row on which the Final Date exceeds 1985-07-20 14:18. You find that the sixth line has a date of 1996, which is the first to exceed 1985. Looking across the row, the BajaN rule applies, and standard time is UTC-8. Finally, look in the third table under the BajaN rule. Once again, find the date range that includes the date in question (1985). That would be the first range, 1976-1985. This says that daylight saving time was in effect from 02:00 of the last Sunday in April until 02:00 of the last Sunday in October for Ensenada in 1985. The date in question does not in fact fall between the last Sunday in April (which was 1985-04-28) and the last Sunday in October, so daylight saving time was not in effect. Therefore, the local time was standard time, which 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.