Is it safe to convert ISO datetime with strtotime

Question

For example

strtotime("2018-12-06T09:04:55");
strtotime("2021-07-09T14:09:47.529751-04:00");

I read in the php manual that ISO dates should be avoided when using strtotime, why ? Should I extract date time from the string before using strtotime.

strtotime() will convert a string WITHOUT a timezone indication as if the string is a time in the default timezone ( date_default_timezone_set() ). So converting a UTC time like '2018-12-06T09:04:55' with strtotime() actually yields a wrong result. In this case use:

<?php
function UTCdatestringToTime($utcdatestring)
{
    $tz = date_default_timezone_get();
    date_default_timezone_set('UTC');

    $result = strtotime($utcdatestring);

    date_default_timezone_set($tz);
    return $result;
}
?>

Which manual are you referring to? The only warning I see in the [docs](https://www.php.net/manual/en/function.strtotime.php) is this one: "The Unix timestamp that this function returns does not contain information about time zones. In order to do calculations with date/time information, you should use the more capable DateTimeImmutable." — Code4R7, Jul 01 '21 at 13:53
All I read is "To avoid potential ambiguity, it's best to use ISO 8601 (YYYY-MM-DD) dates or DateTime::createFromFormat() when possible." and the top post which dates 10 years back says you should be using dots `.` between date components because strtotime() is 'smart' by design. Which reminds us to [EWD 340](https://www.cs.utexas.edu/users/EWD/transcriptions/EWD03xx/EWD340.html). — Code4R7, Jul 04 '21 at 06:25

score 0 · Answer 1 · answered Oct 12 '21 at 10:13

If the date string contains a time zone, strtotime also takes this into account.

$strDate = "2018-12-06T09:04:55 UTC";
$ts = strtotime($strDate);  // int(1544087095)

If the time zone is missing in the date string, the default time zone is used. My Timezone is "Europe/Berlin".

$strDate = "2018-12-06T09:04:55";
$ts = strtotime($strDate);  // int(1544083495)

As a result, we get a different timestamp.

If I want to convert a date string from another time zone into a time stamp, then the best solution is to do it with the DateTime object. There I can enter the correct time zone in the 2nd parameter when creating the object.

$strDate = "2018-12-06T09:04:55";
$dt = new DateTime($strDate, new DateTimeZone('UTC'));
$ts = $dt->getTimeStamp();  // int(1544087095)

Important: If the date string contains a valid time zone, this has priority over the 2nd parameter.

$strDate = "2018-12-06T09:04:55 UTC";
$dt = new DateTime($strDate, new DateTimeZone('Europe/Berlin'));
/*
DateTime::__set_state(array(
   'date' => "2018-12-06 09:04:55.000000",
   'timezone_type' => 3,
   'timezone' => "UTC",
))
*/

The DateTimeZone ('Europe/Berlin') is ignored here.

Since strtotime also accepts a time zone in the date string, the time zone can also be added with a string concatenation.

$strDate = "2018-12-06T09:04:55";
$ts = strtotime($strDate." UTC");  //int(1544087095)

The UTCdatestringToTime function does the same. However, it is not nice to temporarily change the default time zone in the PHP script.

Is it safe to convert ISO datetime with strtotime

1 Answers1