Calculate date of last day of month before a target date - php

Question

I can add any number of months to a date:
strtotime("+ 1 months", '2017-01-31') // result: 2017-03-03
But I want to do this without going to the next month.
in this case I want the result 2017-02-28, that is, the last day of the month before the target month.

Answer 1

There seems to be a lot of overcomplicating in these answers. You want the last day of the month before your target month, which is also always going to be 1 day before the first day of the target month. This can be expressed quite simply:
$months = 1;
$relativeto = '2017-01-31';
echo date(
'Y-m-d',
strtotime(
'-1 day',
strtotime(
date(
'Y-m-01',
strtotime("+ $months months", strtotime($relativeto))
)
)
)
);

Answer 2

Try using the DateTime object like so:
$dateTime = new \DateTime('2017-01-31');
$dateTime->add(new \DateInterval('P1M'));
$result = $dateTime->format('Y-m-d');

Answer 3

Use PHP's DateTime to accomplish this, specifically the modify() method.
$date = new DateTime('2006-12-12');
$date->modify('+1 month');
echo $date->format('Y-m-d');

Answer 4

If the idea here is not to allow the date to overflow into the next month, which PHP does, then you'll have to impose that constraint in your logic.
One approach is to check the modified month against the given month before returning the updated date.
function nextMonth(DateTimeImmutable $date) {
$nextMonth = $date->add(new DateInterval("P1M"));
$diff = $nextMonth->diff($date);
if ($diff->d > 0) {
$nextMonth = $nextMonth->sub(new DateInterval("P{$diff->d}D"));
}
return $nextMonth;
}
$date = new DateTimeImmutable("2017-01-31");
$nextMonth = nextMonth($date);
echo "{$date->format('Y-m-d')} - {$nextMonth->format('Y-m-d')}\n";
//2017-01-31 - 2017-02-28
$date = new DateTimeImmutable("2017-10-31");
$nextMonth = nextMonth($date);
echo "{$date->format('Y-m-d')} - {$nextMonth->format('Y-m-d')}\n";
//2017-10-31 - 2017-11-30
The nextMonth() function in the example above, imposes the constraint of not overflowing into the next month. Note that what PHP actually does is try to find the corresponding day, in the following consecutive number of months added, not just add a given number of months to the existing date. I simply undo this last part by subtracting any additional days beyond the 1 month interval in the function above.
So for example, strtotime("+1 month") for the date "2017-01-31", tells PHP find the 31st day that is +1 month from "2017-01-31". But of course, there are only 28 days in February, so PHP goes into March and counts up 3 more days to compensate.
In other words, it's not just add +1 month to this date, but add +1 month to this date and arrive at the same day of the month as is in the given date. Which is where the overflow happens.
Update
Since you've now made it clear that you actually want the last day of the month (not necessarily the same day of the next month without the overflow provision) you should instead just explicitly set the day of the month.
function nextMonth(DateTimeImmutable $date) {
$nextMonth = $date->setDate($date->format('Y'), $date->format('n') + 1, 1);
$nextMonth = $date->setDate($nextMonth->format('Y'), $nextMonth->format('n'), $nextMonth->format('t'));
return $nextMonth;
}
$date = new DateTimeImmutable("2017-02-28");
$nextMonth = nextMonth($date);
echo "{$date->format('Y-m-d')} - {$nextMonth->format('Y-m-d')}\n";
// 2017-02-28 - 2017-03-31