Adding weeks on to a variable, does not advance the month

The code I am using is

 <button id="btn23" class="btn btn-light" dmx-on:click="check_avilable.load({service: inp_package.value, start_date:[0].start_datetime.addWeeks(1)},true)">Next Week&nbsp;&nbsp;<i class="fas fa-angle-right"></i></button>

Now this work for example through March, however when it gets to the end of March it will not flow through to April. Am I missing something here?

What is this button doing exactly and what date / where are you changing?