php - Removing exactly one month from a date

Question

It seems that you can't use strotime if you need reliable and accurate date manipulation. For example, if the month has 31 days, then it appears that strtotime simply minuses 30 days, not a whole month.

So, for example, if $event["EndDate"] is equal to "2013-10-31 00:00:01", the following code:

echo date("Y/n/j", strtotime('-1 month', strtotime($event["EndDate"]));

Ouputs: 2013/10/1 instead of 2013/09/30.

QUESTION: Now how I know how NOT to do it, is there another, more accurate, way to make PHP subtract (or add) exactly a whole month, and not just 30 days?

Answer 1

The main issue is that 2013/09/31 does not exist so a better approach would be to use first day or last day of the previous month.

$date = new DateTime("2013-10-31 00:00:01");
$date->modify("last day last month");
echo $date->format("Y/n/j"); // 2013/9/30

When date is 2013-10-15

$date = new DateTime("2013-10-15 00:00:01");
$day = $date->format("d");
$year = $date->format("Y");

$date->modify("last day last month");
$month = $date->format("m");

if (checkdate($month, $day, $year)) {
    $date->setDate($year, $month, $day);
}

echo $date->format("Y/n/j"); // 2013-9-15