UTC Time, Timezones, Daylight Savings, and Daylight Savings switchover dates
I'm building an application which will be able to send emails at any specific local time to any place in the world.
For example, my daily schedule (localtime):
I want to be able to execute this code on and Amazon EC2 server in a single location (eg São Paulo, Brasil).
I also know that Toronto is in Eastern Standard Time, (UTC - 5h) , but from March 11, 2012 to November 4, 2012, it is in Eastern Daylight Time (UTC - 4h).
I also know that Perth is in Western Standard Time (UTC + 8h), with no daylight savings.
I also know that Rabat is in Western European Time (UTC), but from April 29,2012 to July 20,2012, and August 19,2012 to Sept 30, 2012 it is in in West European Summer Time (UTC + 1h)
To keep track of these combinations of time zone, daylight savings, et cetera, I will, of course insist that all internal server times be in UTC. However, I need some way to keep track of when and how each time-zone jurisdiction switches time zones because of Daylight Savings or (in the case of Rabat) Ramadan, and then adjust my crontabs to accommodate these changes.
Is there an authoritative web service or set of tables somewhere which would help me keep these timezone changes in sync with my desire to deliver emails at the same local time every day to users in different time zones with different switchover dates for daylight savings?
Most programming languages give you access to timezone conversion functions. The most rudimentary ones only work between UTC and the "local" timezone of the server, so you will need a full-featured one, such as pytz for Python that will let you specify a local time with a timezone name (eg "America/Toronto") and convert it to UTC for you. Given that, you don't need to worry about the UTC offsets of different timezones (including historical offsets if they've changed) nor DST start end end times: the library will take care of it for you. Just make sure you have the latest database, which comes in the tzdata package.
As for your crontab, you're probably best off if the local timezone on the server that runs cron is UTC, that way you can put UTC times directly in the crontab. On the other hand, depending on the volume of events that you have, I would advice just having cron run your code at regular schedules intervals (such as every 5 minutes) and then your code figures out what events need to be triggered based on the current UTC time and the contents of your database. Then it doesn't matter what the timezone of the server is.
Is there an authoritative web service or set of tables somewhere ...
No, there is nothing "authoritative", but there is something close. It's called the TZ database, and it is currently under the oversight of IANA. Its home page is here.
It is also known as tzdata, zoneinfo, timezonedb, tzdb, the Olson Database, or the IANA Time Zone Database.
There are implementations for just about every language and platform you can imagine. You can read more in the tz-link file from the tzdb, and in the timezone tag wiki, here on StackOverflow.
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