The reason for the error is that there is nothing like
Mon, 21 May 2013
from my calendar21
st
May
isTuesday
Dates in the m/d/y or d-m-y formats are disambiguated by looking at the separator between the various components: if the separator is a slash (/), then the American m/d/y is assumed; whereas if the separator is a dash (-) or a dot (.), then the European d-m-y format is assumed.
To avoid potential ambiguity, it's best to use ISO 8601 (YYYY-MM-DD) dates or DateTime::createFromFormat() when possible.
Examples
$strtotime = strtotime("Tue, 21 May 2013 09:10:30 GMT");
echo date("Y-m-d H:i:s", $strtotime),PHP_EOL;
$strtotime = DateTime::createFromFormat("D, d M Y g:i:s O", "Tue, 21 May 2013 09:10:30 GMT");
echo $strtotime->format("Y-m-d H:i:s");
Output
2013-05-21 11:10:30 <- strtotime
2013-05-21 09:10:30 <- datetime