Ethiopian Date conversion(based on JDN)

ethiopic.php

<?php

/**
 * Return a true if the year is a leap year based on the ethiopian calendar.
 *
 * @param int $year the ethiopian date to be checked
 *
 * @return bool
 */
function ethiopian_is_leap_year($year)
{
    if (!is_int($year)) {
        trigger_error(
            __FUNCTION__ . '() expects parameter 1 to be numeric. ' .
            gettype($year) . ' given',
            E_USER_WARNING
        );
    }

    return ($year + 1) % 4 == 0;
}

/**
 * Returns true if the <code>$month, $day and $year</code> passed
 * are a valid date based on the ethiopian calendar.
 *
 * @param int $month Ethiopian month
 * @param int $day Ethiopian day
 * @param int $year Ethiopian year (negative for AD)
 *
 * @return bool
 */
function ethiopian_checkdate($month, $day, $year)
{
    return
        // validate all
        is_int($month) && is_int($day) && is_int($year) &&
        // true if the day is btn 1 - 30
        ($day <= 30 && $day >= 1) &&
        // true if the month is btn 1 - 13
        ($month <= 13 && $month >= 1) &&
        // true if the month is 13 then day is btn 1 - 6
        ($month == 13 ? $day <= 6 : true) &&
        // true if the month is 13 & day is 6 then year must be leap year
        ($month == 13 && $day == 6 ?
            ethiopian_is_leap_year($year) : true);
}

/**
 * Returns the julian date number representation of the
 * given ethiopian date.
 *
 * @param int $month Ethiopian month
 * @param int $day Ethiopian day
 * @param int $year Ethiopian year (negative for AD)
 *
 * @return int
 */
function ethiopiantojd($month, $day, $year)
{
    if (ethiopian_checkdate($month, $day, $year)) {
        trigger_error(
            'ethiopiantojd() expects the date to be valid.' .
            ' check ethiopian_checkdate() first',
            E_USER_WARNING
        );
    }

    return (1723856 + 365) +
    365 * ($year - 1) +
    (int)($year / 4) +
    30 * $month +
    $day - 31;
}

/**
 * Returns the ethiopian date string which is represented by
 * the passed jdn <br />.
 *
 * @param int $jdn the Julian Date Number
 *
 * @return string in a month/day/year format
 */
function jdtoethiopian($jdn)
{
    if (!is_long($jdn)) {
        trigger_error(
            'jdToEthiopian() expects parameter 1 to be numeric. ' .
            gettype($jdn) . ' given',
            E_USER_WARNING
        );
    }

    $r = (($jdn - 1723856) % 1461);
    $n = ($r % 365) + 365 * (int)($r / 1460);
    $year = 4 * (int)(($jdn - 1723856) / 1461) +
        (int)($r / 365) - (int)($r / 1460);
    $month = (int)($n / 30) + 1;
    $day = ($n % 30) + 1;

    return "$month/$day/$year";
}

• Julian Day Number
• PHP JDN implemetation