create_customer_schedule.php

<?php

function isNextDateGreaterThanHalf($frequency, $seasonalEndDate, $nextDate)
{
    $dateDiff = (strtotime($seasonalEndDate) > strtotime($nextDate))
                 ? strtotime($seasonalEndDate) - strtotime($nextDate)
                 : strtotime($nextDate) - strtotime($seasonalEndDate);

    $daysDiff = floor($dateDiff / (60*60*24)); //seconds * mins in hour * hours in day

    return ($daysDiff > ($frequency / 2));
}

function addDays($date, $days)
{
    $strToTimeAddDays = date('Y-m-d', strtotime($date)) . ' + ' . $days . ' Days';
    $date =  date('Y-m-d', strtotime($strToTimeAddDays));

    return $date;
}

function addYears($date, $years)
{
    $strToTimeAddDays = date('Y-m-d', strtotime($date)) . ' + ' . $years . ' Years';
    $date =  date('Y-m-d', strtotime($strToTimeAddDays));

    return $date;
}

function getUpComingServices($lastServiceDate, $seasonalStartDate, $seasonalEndDate, $frequency, $numberOfServices)
{
    echo "Every $frequency days for $numberOfServices services.<br/><br/>";
    echo "Last Service Date:    $lastServiceDate <br/><br/>";
    echo "Start of Season Date: $seasonalStartDate <br/>";
    echo "End of Season Date:   $seasonalEndDate <br/>";

    //list of dates to service customer
    $scheduledDates = [];

    //date before start of season
    if (strtotime($lastServiceDate) < strtotime($seasonalStartDate)) {
        $lastServiceDate = $seasonalStartDate;
    }

    //for each number of services deterimine the next service date
    for ($i = 0; $i < $numberOfServices; $i++) {
        $nextDate = (!isset($nextDate)) ? $lastServiceDate : addDays($nextDate, $frequency);

        //date after end of season
        if (strtotime($nextDate) > strtotime($seasonalEndDate)) {
            $isPastHalf = isNextDateGreaterThanHalf($frequency,
                                                    $seasonalEndDate,
                                                    $nextDate);

            //reset for next season
            if ($isPastHalf === true) {
                $seasonalStartDate = addYears($seasonalStartDate, 1);
                $seasonalEndDate   = addYears($seasonalEndDate, 1);
                $nextDate = $seasonalStartDate;
            }
        }

        //add the date to the list
        $scheduledDates[] = date("Y-m-d", strtotime($nextDate));
    }

    return $scheduledDates;
}


$lastServiceDate = date("2015-11-10");
$frequency = 30;
$seasonalStartDate = date("2015-03-01");
$seasonalEndDate = date("2015-10-31");
$numberOfServices = 25;

$scheduledDates = getUpComingServices($lastServiceDate,
                                      $seasonalStartDate,
                                      $seasonalEndDate,
                                      $frequency,
                                      $numberOfServices);
echo "<pre>";
var_dump($scheduledDates);
echo "</pre><br/><br/>";