determining the time on the user's machine

sneakyimp

i'm working on an event listing. in certain cases, i need to know what the timezone is on the user's machine. i have two questions:

1) if the user enters a zipcode ( a united states postal code) , i would like to determine the time zone for that postal code. can anyone tell me where i might find such data?

2) in the absence of any postal code, how do you determine the time and/or timezone on the user's machine?

NogDog

The only way I can think of is to use some JavaScript on the client side, grab the time info, and then get it to your script: either through an AJAX interface, a form submission, or storing it in a cookie that subsequent pages can access.

Or you could just use the cookie method, and if the cookie value is not set then include a form with a timezone select list from which the user can manually select their timezone.

MarkR

You can determine the time on the client machine by using Javascript. This can fill in a hidden field in a form before you submit it.

If you want to do it without submitting a form, you may be able to change a link URL instead in the page through the DOM.

Alternatively, you could use an AJAX-style request if you wanted (but there is normally no reason to do this).

Another option might be using an ip-to-location database, however these are notoriously unreliable, and mostly only get the country right (Not too good for countries like the USA and Canada with lots of time zones).

Mark

sneakyimp

Thanks guys.

I found this:

<script type="text/javascript">

var d = new Date()
var gmtHours = d.getTimezoneOffset()/60
document.write("The local time zone is: GMT " + gmtHours)

</script>

It occurs to me now that any time I would like to refer to the local time in my PHP code, I must somehow use javascript to communicate the time offset (or just local time) to my server via GET, COOKIE, or POST. I'm not certain but it seems to me that would suggest using one of these two approaches:

1) put some javascript in my headers or templates that attempts to store the user's time offset in a cookie. check the cookie in PHP. If it's empty, what then? I can assume some time zone offset (like GMT) or try to route the user through some cookie test/time zone check page. Sounds pretty dodgy to me.

2) Use javascript to add the current tz offset to every form (or perhaps every link). this also sounds dodgy.

I find it hard to believe that $_SERVER or ENVIRONMENT vars have no vars to determine local time for a given user.

sneakyimp

Ack. This time zone and DST stuff is a lot more complicated than I first imagined. I've solved my first problem - how to highlight the current day on a calendar - by using javascript to tell PHP what day it actually is on the client machine.

HOWEVER, The problem I'm facing now is that I need my server to determine what the current time is (as in the time RIGHT NOW) in a variety of zip codes. Why? I am searching a list of events whose start time is specified locally. These events could happen in any one of 13 different time zones: -12, -11, -10, -9, -8, -7, -6, -5, -4, 1, 9, 10, 11,

I have made some progress. I have a database table full of zip codes. Each zip has a timezone offset value which looks pretty much exactly like what you see in the Windows XP time settings.

For instance, I'm in california so my Win XP time zone setting is (GMT -08:00) Pacific Time (US & Canada); Tijuana

My database has an entry for my zip code that says
zip: 90046
time_zone_offset: -8

They match!

Now the problem is that my server (which is also in california) is currently reporting a GMT offset of 7 hours. This code:

<?
$now = time()
?>
Current GMT Offset in hours: <?=date('O', $now) ?><br>
Current Date and Timezone: <?=date('D, j M Y H:i:s T', $now) ?><br>

Outputs this:

Current GMT Offset in hours: -0700
Current Date and Timezone: Mon, 9 Oct 2006 21:14:18 PDT

I think the culprit here is Daylight Savings Time. On my Windows XP time settings, I have that little box checked. My server is apparently adjusting for DST too. Both Javascript and my server report -7:00 hours GMT rather than -8:00 hours GMT.

I just checked the doc entry for date
http://php.net/date

and noticed that 'I' can tell me whether DST is in effect or not for a date. I think that might offer me a way out of this but I'm having a bit of trouble getting my head around this notion.

Is this correct?

$dst = date('I', $now);
if ($dst == 1) {
  echo 'DST IN EFFECT.  Your offset from GMT will be +1, meaning that california time is -7 instead of -8<br>';
} else {
  echo 'NO DST.  Use the default offset in your database.<br>';
}

I'm still stumbling through the logic. Is there some simple way to keep track of this stuff?

sneakyimp

today the plot thickens. I found that some states in the United States don't observe daylight savings time. So the time offset of a zipcoe relative to the server's time offset can depend not only on the location of the zip code and it's base time zone, but also on whether or not they observe daylight savings time.