Convert Excel date / time to PHP date

convert_excel_to_date.php

<?php

/**
 * Convert date and time from Excel into a PHP DateTime object
 * 
 * Based on similar function from PHPExcel library.
 * 
 * @see https://stackoverflow.com/a/11172688/401980
 * @see http://www.cpearson.com/excel/datetime.htm
 */
function excel_to_date($date_value = 0, $format = null)
{
    /**
     * Number of days between the beginning of serial date-time (1900-Jan-0)
     * used by Excel and the beginning of UNIX Epoch time (1970-Jan-1).
     */
    $days_since_1900 = 25569;

    if ($date_value < 60) {
        --$days_since_1900;
    }

    /**
     * Values greater than 1 contain both a date and time while values lesser
     * than 1 contain only a fraction of a 24-hour day, and thus only time.
     */
    if ($date_value >= 1) {
        $utc_days = $date_value - $days_since_1900;
        $timestamp = round($utc_days * 86400);

        if (($timestamp <= PHP_INT_MAX) && ($timestamp >= -PHP_INT_MAX)) {
            $timestamp = (integer) $timestamp;
        }
    } else {
        $hours = round($date_value * 24);
        $mins = round($date_value * 1440) - round($hours * 60);
        $secs = round($date_value * 86400) - round($hours * 3600) - round($mins * 60);
        $timestamp = (integer) gmmktime($hours, $mins, $secs);
    }

    return $format ? date($format, $timestamp) : $timestamp;
}

echo excel_to_date(44641, 'd.m.Y H:i:s');