Why is the date and time conversion different on different dates? - php

Question

I have four dates with different dates but the same time:
2019-03-02 20:00:00
2019-03-05 20:00:00
2019-03-10 20:00:00
2019-03-18 20:00:00
Those dates and times are in UTC. I want to convert it to a timezone, for example Los Angeles.
Here is the code:
$dateTime = '2019-03-02 20:00:00';
$dt = new DateTime($dateTime, new DateTimeZone('UTC'));
$dt->setTimeZone(new DateTimeZone('America/Los_Angeles'));
$new = $dt->format('Y-m-d H:i:s');
echo $new;
The dates with 2019-03-10 and 2019-03-18 shows up as different times. The answer is because on March 10, it will be 1 hour ahead due to day light savings.
Here is what it comes out with:
2019-03-02 12:00:00
2019-03-05 12:00:00
2019-03-10 13:00:00
2019-03-18 13:00:00
My question is, how can I format my date using the code above and NOT factor in day light saving. I simply want to convert my time without daylight savings. How can I do that? Here is my expected output:
2019-03-02 12:00:00
2019-03-05 12:00:00
2019-03-10 12:00:00
2019-03-18 12:00:00
They are all at 12:00:00.

Answer 1

I simply want to convert my time without daylight savings.
Sorry, but that line of thinking is problematic. Daylight saving time is not something that is optional for the people in the US Pacific time zone (represented by America/Los_Angeles). They don't get a choice of whether to opt in or opt out.
If you are trying to handle some stated business requirement like "Don't worry about DST" - sorry, but you should push back on that. Otherwise your code will generate incorrect results for most of the year. (In the US, the daylight period is actually longer that than standard period!)
However, if what you are really after is not the time in US Pacific time zone, but simply the time at a fixed offset of UTC-8, then instead of America/Los_Angeles, use the time zone Etc/GMT+8 (note that the sign is inverted intentionally). This will use UTC-8 no matter what time of the year you are talking about.
Do keep in mind that the Etc/GMT* zones are primarily intended for edge cases such as time onboard ships at sea. If you simply want to track time in some other part of the world, then chose the appropriate time zone for that location. For example, the Pitcairn Islands use UTC-8 all year long and call it PST, and their time zone identifier is Pacific/Pitcairn.
Refer to the full list of time zones here.
As far as the point you made about 20:00:00 UTC being 12:00:00 PST but 13:00:00 PDT - indeed that is the reality of the situation. If you are tracking 24 hour periods one after another, then there's no way to avoid this fact. Even if instead you tracked "Noon in Pacific Time" each day, then you'd get different corresponding UTC times before and after the transitions.
While UTC days are 24 hours, the transition days in US Pacific time are 23 hours long when we switch from PST to PDT in the spring, and 25 hours long in the fall when we switch from PDT to PST in the fall.
And if you think that is complicated, consider that not every time zone has a 1 hour DST delta. Lord Howe Island in Australia (Australia/Lord_Howe) has a 30 minute delta. They are UTC+10:30 during standard time, and UTC+11 during daylight time.

Answer 2

Since PHP DateTime objects always take daylight savings time into account, you need to make the conversion by subtracting the time difference when daylight savings time is not in effect. This code should do what you want:
$utctime = new DateTime('2019-01-01', new DateTimeZone('UTC'));
$localtime = new DateTime('2019-01-01', new DateTimeZone('America/Los_Angeles'));
$diff = $localtime->diff($utctime);
foreach ($dates as $date) {
$dt = new DateTime($date, new DateTimeZone('UTC'));
$dt->add($diff);
echo $dt->format('Y-m-d H:i:s');
}
Output:
2019-03-02 12:00:00
2019-03-05 12:00:00
2019-03-10 12:00:00
2019-03-18 12:00:00
Demo on 3v4l.org
Update
As was pointed out by #MattJohnson, there exists a timezone Etc/GMT+8 which refers to 8 hours earlier than GMT (regardless of daylight savings time). You can use that timezone to adjust your times:
$west_coast = new DateTimeZone('Etc/GMT+8');
foreach ($dates as $date) {
$dt = new DateTime($date, new DateTimeZone('UTC'));
$dt->setTimeZone($west_coast);
echo $dt->format('Y-m-d H:i:s') . "\n";
}
The output is the same as above. Demo on 3v4l.org