I reproduced the issue with more descriptive code: https://3v4l.org/57h2S

```
<?php

var_dump(date_default_timezone_get());

for ($y = 1970; $y <= 2020; $y++) {
    $ts = strtotime((string) $y);
    $date = date("Y-m-d H:i:s", $ts);
    $isL = date("L", $ts);
    print "$y :: $date :: $isL\n";
}
```

The problem here is the use of strtotime(). strtotime() is a "best efforts guess" based on a set of rules and, as in this example, can unexpectedly change its interpretation for similar values.

Here we can see at a value of "2000", it starts interpreting the value as time instead of year.

The solution is to use DateTime::createFromFormat() instead - as the format is explicitly specified, the interpretation will not stray from what you expect: https://3v4l.org/Z38gl

```
<?php

var_dump(date_default_timezone_get());

for ($y = 1970; $y <= 2020; $y++) {
    $dt = DateTime::createFromFormat("Y", $y);
    $date = $dt->format("Y-m-d H:i:s");
    $isL = $dt->format("L");
    print "$y :: $date :: $isL\n";
}
```

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

This is expected. Don't use strtotime() for this, or, if you must, specify a full date in the form of "{$y}-01-01".