Calculating amount of months between dates

Question

Hi,

How would you go about calculating the amount of months between two arbitrary dates? Given that even if just one day falls on a month, it is considered a full month.

Examples:

• 2010-01-01 - 2010-03-31 = three months
• 2010-06-15 - 2010-09-01 = four months

Et cetera. I thought of just dividing the difference of timestamps with 2592000 (average number of seconds in a month) but that seems hacky and prone to errors. And I'd like to keep it as fast as possible (needs to run thousands of times quick), so I guess using strtotime isn't optimal either?

Answer 1

If I am reading your question correctly, you would want to return "2" for January 31st and February 1st, because it spans both January and February, even though they are only 1 day apart.

You could work out (psuedocode):

monthno1 = (date1_year * 12) + date1_month;
monthno2 = (date2_year * 12) + date2_month;

return (monthno2 - monthno1) + 1;

This assumes that the second date is the later date.

Answer 2

Assuming the dates are in a known format:

function getMonths($start, $end) {
    $startParsed = date_parse_from_format('Y-m-d', $start);
    $startMonth = $startParsed['month'];
    $startYear = $startParsed['year'];

    $endParsed = date_parse_from_format('Y-m-d', $end);
    $endMonth = $endParsed['month'];
    $endYear = $endParsed['year'];

    return ($endYear - $startYear) * 12 + ($endMonth - $startMonth) + 1;
}

This gives:

print(getMonths('2010-01-01', '2010-03-31')); // 3
print(getMonths('2010-06-15', '2010-09-01')); // 4