PeriodMultiplierTrait.php

<?php

use Carbon\Carbon;

/**
 * Trait PeriodMultiplier
 * Used in cases when class has
 */
trait PeriodMultiplierTrait
{
    /**
     * Calculate float amount of months
     *
     * @param Carbon $startDate
     * @param Carbon $endDate
     *
     * @return float
     */
    protected function calculatePeriodMultiplier(Carbon $startDate, Carbon $endDate) : float
    {
        $startDay = $startDate->day;
        $endDay = $endDate->day;
        $startDateBase = (clone $startDate)->addDays(1 - $startDay);
        $endDateBase = (clone $endDate)->addDays(1 - $endDay);

        if ($startDay === 1 && $endDay === $endDate->daysInMonth && $startDate->month === $endDate->month) {
            $diffInYears = $startDate->diff($endDate)->format('%r%y');

            return $diffInYears * 12 + 1;
        }

        $startDate->setTime(0, 0, 0);
        $endDate->setTime(0, 0, 0);

        if ($startDate->day <= $endDate->day && ($startDate->day !== $startDate->daysInMonth || $endDate->day !== $endDate->daysInMonth)) { //strange PHP DateInterval behaviour
            $monthDiff = $startDate->diffInMonths($endDate, false); //TODO update this
        } else {
            $monthDiff = $startDate->diffInMonths($endDate, false) + 1;
        }

        $amount = $monthDiff -
            ($startDay - 1) / ($startDateBase->diffInDays((clone $startDateBase)->addMonth())) +
            ($endDay - 1) / ($endDateBase->diffInDays((clone $endDateBase)->addMonth()));

        return round($amount, 3);
    }
}