Rules currently in force

Time zone history

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

Example: When it was 14:18 (or 2:18 p.m.) local time on 1945-09-03 (September 3, 1945) in Ponta Delgada, Portugal, what was the UTC?

Ponta Delgada is in the Azores. From the first table, it's in the Atlantic/Azores time zone. In the second table, look in the block of rows for Atlantic/Azores. Among them, look for the first row on which the Final Date exceeds 1945-09-03 14:18. You find that the third line in the block has a date of "1966-04-03 02:00", which is the first to exceed 1945. Looking across the row, the Port rule applies, and standard time is UTC-1. Finally, look in the third table under the Port rule. Once again, find the date range that includes the date in question (1945). The 1945 row is the fourth in a group of rows that are all included under "Standard Time, with Summer Time = Standard Time + 1:00 during period (1), superseded by Midsummer Time = Standard Time + 2:00 during period (2):". The rule says that Summer Time was in effect in Ponta Delgada from 23:00 local time on March 10, 1945 to 00:00 local time on October 28, 1945, and Midsummer Time from April 21, 1945 at 23:00 to August 26, 1945 at 00:00. The date in question occurred during summer time but not during midsummer time. Therefore, local time was one hour ahead of standard time, making it the same as UTC. The answer is 1945-09-03 14:18 UTC.

Time zone names

Portugal is a member of the European Union, which has standardized names and abbreviations for time zones. According to the EU, any zone where the standard time is UTC should be called "Western European Time" (WET), or, when daylight saving is in effect, "Western European Summer Time" (WEST). A zone where the standard time is UTC+1 is called "Central European Time" (CET), or, when daylight saving is in effect, "Central European Summer Time" (CEST). The EU hasn't defined a name for a zone with a standard time of UTC-1, such as Atlantic/Azores.

The tz database contains names and abbreviations for the Portuguese time zones. They were created by a contributor. Don't assume that these names, or anything like them, have ever been used officially in Portugal.
For Europe/Lisbon, use Lisbon Mean Time (LMT) before 1911-05-24; from then on, use the EU names, with Western European Midsummer Time (WEMT) when Midsummer Time is in effect (1942-1945).
For Atlantic/Azores, use Horta Mean Time (HMT) before 1911-05-24; from then on, use Azores Time (AZOT), Azores Summer Time (AZOST), or Azores Midsummer Time (AZOMT), except when the Azores observed Western European Time from 1992-09-27 to 1993-03-28.
For Atlantic/Madeira, use Funchal Mean Time (FMT) before 1911-05-24; from then until 1966-04-03, use Madeira Time (MADT), Madeira Summer Time (MADST), or Madeira Midsummer Time (MADMT); then use the EU names.