Description:
------------
I'm working on a payment processor which unfortunately has to take several types of date input, as a result I need to test if a date I'm being passed is already a unix timestamp. For the most part strtotime() will return false when passed a timestamp. There are a few edge cases where it will return another timestamp sometime after year 2400 or before 2000. If this is actually the expected behavior it might be worthwhile to throw a warning in the documentation about passing strtotime() unix timestamps

Test script:
---------------
$errors = 0;
$end_date = strtotime('January 1st 2020');
$start_date = strtotime('January 1st 2001');
$bad_years = array();
while ($start_date < $end_date){
	if (strtotime($start_date) !== false){
		echo date ('Y-m-d', $start_date) . ' generates ' . strtotime($start_date) .  '  ' . date('Y-m-d', strtotime($start_date)) . "\n";
		$bad_years[] = date('Y', strtotime($start_date));
		$errors++;
	}
	$start_date += 86400;
}
print_r(array_unique($bad_years));
echo "$errors errors generated\n";

Expected result:
----------------
sam.heller@sam:/var/www/development/api/app/scripts$ php test.php 
Array
(
)
0 errors generated
sam.heller@sam:/var/www/development/api/app/scripts$ 


Actual result:
--------------
sam.heller@sam:/var/www/development/api/app/scripts$ php test.php 
2001-09-14 generates 51442009244  3600-02-18
2001-09-15 generates -62163017947  0000-02-18
2001-09-25 generates 64064790099  4000-02-18
2001-09-26 generates -49540237092  0400-02-18
...
...
...
2019-05-22 generates -24294654070  1200-02-18
2019-05-23 generates 177669838738  7600-02-18
2019-06-02 generates -11671873215  1600-02-18
2019-06-03 generates 190292619593  8000-02-18
Array
(
    [0] => 3600
    [1] => 0000
    [2] => 4000
    [3] => 0400
    [4] => 6800
    [5] => 0800
    [6] => 7200
    [8] => 1200
    [9] => 7600
    [11] => 8000
    [12] => 4400
    [14] => 8400
    [15] => 4800
    [17] => 5200
    [18] => 1600
    [20] => 2000
    [22] => 2400
    [23] => 8800
    [25] => 9200
    [26] => 5600
    [28] => 9600
    [29] => 6000
    [31] => 6400
    [32] => 2800
    [34] => 3200
)
1007 errors generated
sam.heller@sam:/var/www/development/api/app/scripts$