Mexico time zones, and the hour a house keeps

How many time zones does Mexico have, and which is which?

Four. Article 3 of the Ley de los Husos Horarios divides the country into Zona Centro on the 90th meridian, Zona Pacífico on the 105th, Zona Noroeste on the 120th and Zona Sureste on the 75th, and the assignment is made by state rather than by town.

Zona Centro is the default and covers the greater part of the national territory. Zona Pacífico is named state by state — Baja California Sur, Nayarit with the exception of the municipality of Bahía de Banderas, Sinaloa and Sonora — and the exception inside it is worth noticing, because it shows what kind of list this is: a municipality can be lifted out of its state's zone by name, and one has been.

Zona Noroeste is the state of Baja California alone. Zona Sureste is the territory of the state of Quintana Roo, which is why a house in Tulum or in Cancún keeps an hour that nothing else on the Caribbean side of the country keeps. Islands, reefs and cays fall into the meridian their geographical situation gives them.

The practical reading of article 3 is short. Look at the state on the deed, not at the map: two houses four hundred kilometres apart can keep the same hour, and two houses far closer together can keep different ones, because the line that separates them is a state boundary written into a statute.

Does Mexico still change its clocks?

Not where any of these houses stand. Article 4 establishes one standard hour per zone and says that a seasonal hour applies only to the states and municipalities of the northern border.

The border is not described in general terms: article 5 names it. Eleven municipalities in Coahuila de Zaragoza, Anáhuac in Nuevo León and ten in Tamaulipas keep the seasonal hour of the 75th meridian; three municipalities of Chihuahua keep the 90th; the State of Baja California, together with Janos, Ascensión, Juárez, Praxedis G. Guerrero and Guadalupe in Chihuahua, keep the 105th. None of the places this office works in appears on that list, and the list is the whole of the exception.

There is a detail in how the law does it that matters more than it looks. The statute does not instruct anyone to move a clock forward: it assigns those municipalities a different meridian for the season, in force from two in the morning on the second Sunday of March until two in the morning on the first Sunday of November. The hour changes because the meridian changes, and everywhere else the meridian is fixed.

So the answer to whether Mexico still changes its clocks is: a strip of the north does, twice a year, and the rest of the country does not at all. For an owner abroad that is the good half of the news, and the next section is the other half.

What changed with the Ley de los Husos Horarios?

It ended the nationwide seasonal hour. Published in the Diario Oficial de la Federación on 28 October 2022, the law entered into force on 30 October 2022 and abrogated the Ley del Sistema de Horario published on 29 December 2001.

It also repealed the instrument that had actually moved the clocks. The transitional articles abrogate the decree establishing the seasonal hour published on 1 March 2002, and all its subsequent reforms, which is the document under which the whole country used to change in spring and autumn. A house bought before 2022 and looked after by the same people since has therefore lived through a change of regime that nothing in the house records.

One month after publication the law was amended once. The reform of 29 November 2022 rewrote the second fraction of article 5 and added the third, which is how Baja California and those five Chihuahua municipalities came to be inside the seasonal border. Nothing in that reform touched the four zones of article 3.

For an owner the consequence is a single sentence, and it is worth keeping. Since the end of October 2022 a house in Quintana Roo, in Guanajuato, in Guerrero or in Baja California Sur keeps one hour for the whole year, and the only clock in the arrangement that still moves is the owner's.

Which Mexican zone matches which US zone through the year?

The three meridians these houses fall on each line up exactly with an American standard hour in winter, because both statutes count the same way: fifteen degrees of longitude to the hour, on the meridian system Mexican law adopts by accepting the agreements of the International Meridian Conference of 1884. From March to November the American zone advances and the Mexican one does not, so what was a match becomes a gap of exactly one hour.

The American offsets come from the same kind of source. Title 15 of the United States Code sets the standard time of the second zone at Coordinated Universal Time retarded by 5 hours, the third by 6 and the fourth by 7, and the federal regulations give those ordinals the names people actually use: the second zone is the eastern standard time zone, the third the central and the fourth the mountain.

Read the last two columns together and the table says one thing. For about four months of the year an American owner and the house are on the same clock, and for the other eight there is one hour between them — not because the house moved, but because the owner did.

Why does the difference move when Mexico does not?

Because the change happens on the American side of the arrangement. Federal law advances the standard time of each United States zone by one hour during the period commencing at 2 o'clock antemeridian on the second Sunday of March and ending at 2 o'clock antemeridian on the first Sunday of November.

Those are the same two Sundays the Mexican statute uses. The seasonal hour of the northern border runs from two in the morning on the second Sunday of March to two in the morning on the first Sunday of November, which is the American calendar kept for the strip of the country that trades across the line every day — and for nowhere else. The one calendar Mexico did not abandon applies to none of the houses on this site.

So the gap opens on a Sunday in March and closes on a Sunday in November, both decided by a law that does not apply where the house stands. Nothing in Mexico announces either date, because in Mexico neither date is an event.

There is one way the map itself can change, and it is slow and public. Article 6 lets a state congress send the federal Congress an initiative to modify the time zone or seasonal hour that applies to the state or municipality, and where a state borders a foreign territory on a different zone the federal Congress has ninety days to make the change. The hour a house keeps is therefore worth checking against the state on its deed rather than remembering from the year it was bought.

What does that mean for a call, a check-in and a monthly report?

It means the hour has to be written down rather than assumed, and re-read twice a year. A house notices none of this; a call, a guest arriving after dark and a report with a date on it notice all of it.

None of this is difficult. It is simply the kind of thing that is obvious to whoever is standing in the house and invisible to whoever is not, and the difference between the two is one line in a statute that most people have no reason to read twice.