Determining whether time/place is using DST

[jennyp]

I have 6 variables that represent a point in time and space, like this:

$Minutes = '05'; // i

$Hour = '08'; // H

$Date = '1'; // j

$Month = 'May'; // F

$Year = '1956'; // Y

$TimeZone = 'America/New_York'; // e

Is it possible to use these together to find out whether that time/place was using daylight savings or not?

I'm running PHP version 5.2.10.

.

[anakadote]

Here's a link that describes the start and end dates for DST through what years. So one way to do it would be to write a function that would compare your date against the info provided:

Ex:

function isDST($date){
  if($date < [date when DST began]){ // Predates the inception of DST
    return false;
  } else if($date >= [date when DST began] && $date < [next DST milestone]){
    if($date >= [DST start date for this range] && $date <= [DST end date for this range])
      return true;
    else
      return false;
  } else if...
}

You could also try using mktime(), which the PHP manual states may cause unexpected results.

[NogDog]

This seems to work, but I'm not sure whether or not it correctly deals with historical times like that when the daylight time transitions were different than they are now.

$mytz = date_default_timezone_get(); // save current TZ
date_default_timezone_set($TimeZone);
$ts = strtotime("$Date $Month $Year $Hour:$Minutes");
$dstFlag = date("I", $ts);
printf(
   "&#37;s is <b>%s</b> time",
   date('Y-m-d H:i e', $ts),
   date('I', $ts) ? 'Daylight' : 'Standard'
);
date_default_timezone_set($mytz); // back to orig TZ

[jennyp]

Thanks ... I got close to that! But now I'm uncertain whether I should ever use it, because of the uncertainty you mention.. I had an idea that historical dst values were included in this version of PHP with the Olson tz stuff.. but for my example it's a negative timestamp, and maybe I was wrong to think it could be done.

[Weedpacket]

Here's a wee experiment:

date_default_timezone_set('America/Los_Angeles');

echo gmdate('Y-m-d H:i:s', strtotime('1918-03-10'));
echo "\n";
echo gmdate('Y-m-d H:i:s', strtotime('2009-03-10'));

According to the Olsen database, one of these 10th March dates is in Daylight Saving while the other isn't.

[jennyp]

That could be correct for all I know! But is my negative timestamp actually making any sense with that 1956 example?

[NogDog]

jennyp;10943947 wrote:

That could be correct for all I know! But is my negative timestamp actually making any sense with that 1956 example?

Yes: zero (0) is 1970-01-01 00:00:00 GST. A negative timestamp indicates that many seconds prior to that time, a positive number that many seconds after. On systems which store it as a signed 32-bit integer, that gives you a span of about 68 years in each direction. (This is why you may have heard of the potential for a Y2K38 bug if such systems are not using a 64-bit architecture by then, or some other means of representing/storing timestamps.)

[jennyp]

Thanks for the help. I think I've got it as far as it will go.

[Weedpacket]

jennyp wrote:

That could be correct for all I know! But is my negative timestamp actually making any sense with that 1956 example?

That's why my test used 1918...