Capturing a start date and number of days

booking-form.liquid

{% comment %}
BookThatApp auto installed snippet. Changes made to this file will be lost if installed again.
{% endcomment %}

{% if product.metafields.bookthatapp.product_id %}
<div class="selector-wrapper">
    {% capture attribute %}booking-start{% endcapture %}
    <label for="{{ attribute }}">Rental Start Date:</label>
    <input id="{{ attribute }}" type="text" name="properties[{{ attribute }}]" class="datepicker bta required bta-load-enable styled-input" autocomplete="off" size="12" data-handle="{{ product.handle }}" data-variant="{{ product.variants.first.id}}" data-mindate="{{ product.metafields.bookthatapp.lead_time }}" disabled="disabled" />
    <label id="booked-out"></label>
</div>

{% capture attribute %}booking-finish{% endcapture %}
<input id="{{ attribute }}" type="hidden" name="properties[{{ attribute }}]" />

<script type="text/javascript">
function isAvailable(date) {
    var result = true;
    
    bta.checkAvailability(date, $('#booking-start'), {
        blackedout: function() {
            result = false;
        },
        unscheduled: function() {
            result = false;
        },
        availability: function(capacity, bookingCount) {
            if (capacity == 0) {
                result = false;
            } else if (capacity > 0 && bookingCount >= capacity) { // product is booked out
                result = false;
            }
        }
    })
    
    return result;
}

function checkAvailability(start, qty) {
    var available = true;
    
    for (var i = 0; i < qty && available; i++) {
        var proposed = new Date();
        proposed.setDate(start.getDate() + i);
        available = isAvailable(proposed);
    }
    
    return available;
}

function updateFinishDate() {
    var qty = parseInt($('#quantity').val(), 10),
        start = $('#booking-start').datepicker('getDate'),
        finish = start,
        available = checkAvailability(start, qty);
    
    if (!available) {
        $('#booked-out').text('The product is not available at the requested time.');
        $('#add').attr('disabled', 'disabled');
    } else {
        $('#booked-out').text('');
        $('#add').removeAttr('disabled');
    }
        
    finish.setDate(start.getDate() + qty);
    var parts = bta.YMDDateString(finish).substring(0, 10).split('-');
    
    $('#booking-finish').val(parts[1] + "/" + parts[2] + "/" + parts[0]);
}

$(document).ready(function() {
    $('#quantity').change(function() {
        updateFinishDate();
    });
})

var bta = {
    product_id: {{ product.id }},
    callbacks: {
      dateSelected: function(inst, date) {
        updateFinishDate();
      }
    }
}

</script>
{% endif %}