default parameter for class method

I have this function (which works fine):

    String Esper::rtcTimeString() {
      DateTime now = rtc.now();
      char buf[] = "MM/DD/YY hh:mm:ss";
      return now.toString(buf);
    }

I'd like to have a default value of "MM/DD/YY hh:mm:ss" but also pass an argument to a parameter such as "MM/DD/YYYY" if I want to override this default behavior.

Typically, in the definitions .h file I would do something like:

    String rtcTimeString(char ts[] = "MM/DD/YY hh:mm:ss");

and subsequently in the .cpp:

    String Esper::rtcTimeString(char ts[]) {
      DateTime now = rtc.now();
      return now.toString(ts);
    }

However that results in the same returned time string every time.

e.g.

1/6/2020 12:01:12
1/6/2020 12:01:12
1/6/2020 12:01:12

How can I implement a parameter such that the functionality is the same as in the first code block?