Custom Calculation to set date based on another date field

I'm working with two date fields.  When a date is entered into the first field(LeaseStart_af_date), I need the second field(BillStart_af_date) to display the 1st day of the month following.

So, if Lease Start date is 11/07/2018

Bill Start should be 12/01/2018

I'm using a custom calculation script for BillStart_af_date but it is not calculating.  The field 'LeaseStart_af_date' does not use any calculation.  Neither field is validated.  Both fields have format set to 'Date mm/dd/yyyy'.  I'm sure its an error with my script.  Any help would be appreciated.

var d = util.scand("mm/dd/yyyy",this.getField("LeaseStart_af_date").valueAsString);

if (d.getMonth() == 11) {

    d.setMonth(0);

    d.setFullYear(d.getFullYear() + 1);

}

else {

    d.setMonth(d.getMonth() + 1);

    d.setDate(1);

}