This is not a bug. This is standard behaviour for date arithmetic when the target 
date is impossible. There is an explanation from the GNU tools here:

http://www.gnu.org/software/tar/manual/html_chapter/Date-input-
formats.html#SEC125

"-1 month" is literally subtracting one month from the date. 2013-07-31 becomes 
2013-06-31 which overflows to 2013-07-01. Likewise "2013-03-31 -1 month" becomes 
2013-03-03.

If you want the last day of the previous month then the method I know (using 
only strtotime) is going to the beginning of the month, subtracting a month, and 
going to the last day:

strtotime("2013-07-31, first day of, -1 month, last day of")
Keep an eye on http://php.net/datetime.formats.relative

But it's much easier with mktime(0, 0, 0, month, 0, year).

I made fix for this, in case somesone has same situation:


$today_date = '2013-07-31'; 
$current_month = date("n", strtotime("$today_date"));
$prev_full_date = $today_date.' 23:59:59';
$prev_full_date = date("Y-m-d H:i:s", strtotime("$prev_full_date -1 month"));

// 2013-07-31 => 2013-07-01 => 2013-06-30
$prev_year = date("Y", strtotime("$today_date -1 month"));
$prev_month = date("m", strtotime("$today_date -1 month"));
$days_in_prev_month = date("t", strtotime("$today_date -1 month")); // Dienu 
skaits 
iepriekšējā mēnesi date(t)
if(date("n", strtotime($prev_full_date)) == $current_month){
	$prev_full_date = date("Y-m-d H:i:s", strtotime("$prev_full_date -1 
day"));
}
else{
	$prev_full_date_tmp = "$prev_year-$prev_month-$days_in_prev_month 
23:59:59";
	$prev_full_date = date("Y-m-d H:i:s", strtotime($prev_full_date_tmp));
}