Is it safe to convert ISO datetime with strtotime - php

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;
}
?>

Answer 1

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.