Javascript function to add days, is adding up a month ahead

Question

I’m with this Javascript function that’s adding up the days on a date, the days it sums up right, only instead of it adding up 24/10/2017 + 2 days = 26/10/2017, He plays a month forward, then he gets like this 24/10/2017 + 2 days = 26/11/2017. Go on like I’m doing:

 now = new Date;
 var dia_atual = now.getDate();
 var dia_escolha = document.getElementById("<%= txtDiaVencimento.ClientID %>").value;
 if ($("#<%=txtTipodePlano.ClientID %>").val() == "MENSAL") {
     if (parseInt(dia_escolha) > parseInt(dia_atual)) {
         var total_dias = dia_escolha - dia_atual;
         var data_tolerancia;
         var tol = (document.getElementById("<%= txtDiaVencimento.ClientID %>").value);
         var data = toDate(document.getElementById("<%= txtDataInicio.ClientID %>").value);

         function toDate(data) {
             let partes = data.split('/');
             return new Date(partes[2], partes[1], partes[0]);
         }
         data.setDate(data.getDate() + total_dias);
         document.getElementById("<%= txtVencimentoC.ClientID %>").value = data.format("dd/MM/yyyy HH:mm:ss");
         data.setDate(data.getDate() + parseInt(total_dias));
         document.getElementById("<%= txtDataTolerancia.ClientID %>").value = data.format("dd/MM/yyyy HH:mm:ss");
     }

Answer 1

Javascript Date object month has the period between 0 (January) and 11 (December) and not between 1 and 12.

So, in your calculation, you can subtract the month from the user input in -1.

return new Date(partes[2], (partes[1] - 1), partes[0]);

Additional links:

MDN - Javascript Date

Problem with time display