dst - PHP daylight saving time detection

Question

I need to send an email to users based wherever in the world at 9:00 am local time. The server is in the UK. What I can do is set up a time difference between each user and the server's time, which would then perfectly work if DST didn't exist.

Here's an example to illustrate it:

• John works in New York, -5 hours from the server (UK) time
• Richard works in London, UK, so 0 hour difference with the server.

When the server goes from GMT to GMT +1 (BST) at 2:00am on a certain Sunday, this means that John now has a -6H time difference now.

This scenario I can still handle by updating all the users outside the server's local time, but once I've moved forward/backward the time of all the other users, I still need a way to detect when (time and date) the users living outside the UK will (or will not) change their local time to a probable DST one.

I need a PHP method to know/detect when other parts of the world will enter/exit DST.

Answer 1

Do you need to know all the details of DST transition yourself? or do you just need to know when is 9:00 am in a given timezone?

If it's the latter, PHP can use your operating system's timezone database to do that for you. The strtotime() function is remarkably good at "figuring out" what you mean:

echo strtotime("today 9:00 am America/New_York");  // prints "1306501200"
echo strtotime("today 9:00 am Europe/London");     // prints "1306483200"

Just make sure you're using one of the PHP supported timezones.