How can I add dynamic number of days to a Carbon date object

Question

Please how can I manipulate Carbon date object to add the number of days in the current month to the date object itself.
I have the following html form:

<form method="POST">
   @csrf
   <input type="hidden" name="plan" id="plan" />
   <div class="form-group">
     <h5>@lang('Enter Invest Amount')</h5>
     <div class="input-group-append">
       <input type="text" name="amount" class="form-control" />
         <span class="input-group-text">{{ $general->cur_text }}</span>
     </div>
     <br>
     <h5>@lang('Select Investment End Date')</h5>
     <div class="input-group-append">
       <input type="text" name="investmentEndDate" class="investmentEndDate form-control"/>
     </div>
   </div>
   <div class="form-group">
     <button type="submit" class="btn btn--base" style="width:100%;">@lang('Invest')</button>
   </div>
</form>

Then Javascript:

const date = new Date();
const daysInTheMonth = new Date(date.getFullYear(), date.getMonth(), 0).getDate();
$(function () {
   $('.investmentEndDate').datepicker({
       minDate: daysInTheMonth
   });
   $('.planModal').on('click', function() {
     $('#depoModal #plan').val($(this).data('plan'));
   });
});

then in my controller:UserController class, I have :

public function postplan(Request $request)
{

    $today = Carbon::now();
    $endDate = $today->addDays($today->daysInMonth);//emphasis here - it keeps returning today's date

    $request->validate([
        'plan' => 'required|numeric|exists:plans,id',
        'amount' => 'required|numeric|min:1',
        'investmentEndDate' => 'required|date|after_or_equal:'.$endDate
    ]);

    var_dump($endDate);//returns today's date 
    
    //other code ....
 }

When I submit the form, the date is not working as expected. I want to add the number of days in the current month to the date object but it keeps returning today's date instead of current date + 31 days which should give me 2021-09-14.
Please what am I doing wrong?

Answer 1

You dont need to assign it in another variable. addDays() modifies the current Carbon object.

public function postplan(Request $request)
{

    $today = Carbon::now();
    $today->addDays($today->daysInMonth);

    $request->validate([
        'plan' => 'required|numeric|exists:plans,id',
        'amount' => 'required|numeric|min:1',
        'investmentEndDate' => 'required|date|after_or_equal:'.$endDate
    ]);

    var_dump($today);// this will be at the date of one month later
    
    //other code ....
 }