Number of decimal months between two dates

Question

How do you find the number of decimal months between two dates in php?

I tried the code below:

$date1 = '2010-01-25';
$date2 = '2010-02-20';

$ts1 = strtotime($date1);
$ts2 = strtotime($date2);

$year1 = date('Y', $ts1);
$year2 = date('Y', $ts2);

$month1 = date('m', $ts1);
$month2 = date('m', $ts2);

$day1 = date('d', $ts1);
$day2 = date('d', $ts2);

$diff = (($year2 - $year1) * 12) + ($month2 - $month1);

but it only gives me whole months. I want with decimals. Like 1.5 months or 1.4 months.

Answer 1

First off use DateTime::diff to compare your dates. This will give you a DateInterval result object.

Then I would do something like this:

$months = ($interval->y * 12) + $interval->m; // Total number of whole months
$months += number_format($interval->d / 30, 1); // Add the month fraction

Working example: http://3v4l.org/f6n3r

Answer 2

Dividing amount of days by 30 days is an easy implementation. It is not acceptable for sensitive calculations such as invoices or other finance related stuff.

There is an awesome DateTime extension called Carbon

$start = Carbon::create(2010, 1, 25, 0, 0, 0);
$end = Carbon::create(2010, 2, 20, 0, 0, 0);

echo $start->floatDiffInMonths($end); // "0.90437788018433"

It considers amount of days in a month, leap year and other things.

Keep in mind that calculation happens including time, so if you add 24 hours to $end, then 20 Feb 2010 will also be included:

$start = Carbon::create(2010, 1, 25, 0, 0, 0);
$end = Carbon::create(2010, 2, 20, 24, 0, 0);

echo $start->floatDiffInMonths($end); // "0.94009216589862"

It also supports daylight saving times in different timezones, for that, you'll need to use floatDiffInRealMonths