IF函数是否正在删除javascript中的日期对象?

我花了一个小时寻找答案,尝试了不同的事情,所以我感谢这里的任何帮助。

下面的代码非常适合查找某人的B部分生效日期。然而,当某人的生日真的在一个月的第一天时,“if”函数get就被使用了,我不再能够格式化和写入日期。这就像“partB_eff”不再是日期对象一样。(我是新手,所以我可能只是在编造这个角色。)

我得到一个错误“TypeError:partB_eff.toLocaleDateString不是自动填充6_步骤_检查表中的函数(代码:24:27)”

我该如何解决这个问题?

let birthday = new Date(e.values[2]);
    //this is a date entered from a google form
let bdayCopy = new Date(birthday);
    //I created this since I'll be using .setMonth(), and I don't want to change the original date of the birhtday
let bday65 = new Date(bdayCopy.setMonth(bdayCopy.getMonth()+780));
    //finds the 65th birthday
let partB_eff = new Date(bdayCopy.setDate(01));
    //find's the Medicare part B effective date (the 1st of the month someone turns 65)

if(birthday.getDate()==1){
    partB_eff = partB_eff.getMonth-1;
    //if the person's birthday is really on the 1st of the month, the part b effective date is the 1st of the month prior. partB_eff must be converted
  }

partB_eff = partB_eff.toLocaleDateString('en-us',{year:"numeric",month: "short",day:"numeric"});
    //format partB_eff so that it looks nice on paper