A Need To Check For Daylight Saving Time - php

Question

What i am trying to archive is what time exactly a post was posted based on the users default timezone via there IP.
What i am worried about is daylight saving time so say if it's 1:46PM now then if daylight saving time in effect it might still post a update as 1:46PM instead of the exact time 3:46PM
The question is does PHP automatically check against that? or is there anything I need to do to see if daylight saving is in effect or not
$timezone = '+0:00';
$timezone = preg_replace('/[^0-9]/', '', $timezone) * 36;
$timezone_name = timezone_name_from_abbr(null, $timezone, true);
date_default_timezone_set($timezone_name);
echo date('D d M Y H:i:s');
Thanks :)
Just checking as everything needs to be 100% accurate

Answer 1

The answer is yes, PHP can give you the precise time based on timezone, regardless of DST. For example, if a user's IP tells you that they're in New York, you could do
$date = date_create('now America/New_York');
to create a DateTime object with the current time in New York.
echo $date->format('h:i:s');
Incidentally, DST is currently in effect, and this code prints the correct current time of 12:21:32.
EDIT
In response to your follow-up question, yes, the time that you save is the time that you will retrieve. For example,
$timestamp = (int) $date->format('U');
would save the absolute unix time of 12:21:32 (or whenever you run this code), regardless of timezone or DST. Another DateTime object will yield the same time:
$retrieved = new DateTime;
$retrieved->setTimestamp($timestamp);
echo $retrieved->format('h:i:s'); // outputs '12:21:32'
Hope that helps.
EDIT 2
To answer your next question, it is indeed possible to adjust a DateTime you've saved to a different timezone. First, set your script's default timezone to UTC.
date_default_timezone_set('UTC');
Then, you can save your timestamps as shown above (except, don't specify a timezone), and when you retrieve them, you can adjust them to where your users are:
echo $retrieved->format('h:i:s'); // Outputs UTC time of 5:21:32
$retrieved->setTimezone(new DateTimeZone('America/New_York'));
echo $retrieved->format('h:i:s'); // Outputs correct New York time of 12:21:32
Even after DST ends, this code would still display 12:21:32 (or, again, whatever time at which you run it)
Hope that helps, again!
EDIT 3
To address your most recent question, you can always just adjust the DateTime object based on the user's settings. For example, if they decide to override DST to off, then you should
// Change your default timezone to that of your user
date_default_timezone_set('America/New_York');
// Check for daylight savings time with date('I')
if (date('I', $timestamp) == 1) $retrieved->modify('-1 hour');
And that should do it!