Problem while trying to spy on moment.js function

I've been trying for a project to implement tests with jasmine on some Angular code.

Here is how I tried to make it work

beforeEach(() => {
        input = [
            "2020-09-30T21:00:00Z",
            "2020-09-30T21:00:20.2Z",
            "2020-09-30T21:00:00.0002Z",
            "2020-09-30T21:00:00.200002Z"
        ];

    });
    it('sould output a valid date format corresponding to entry date', () => {
        spyOn(moment().tz, 'format').and.callThrough()

        var output = [
            "23:00:00",
            "23:00:20.200",
            "23:00:00.000.2",
            "23:00:00.200.002"
        ];
        expect(myService.myFunction(input[0], "Europe/Paris")).toEqual(output[0]);
        expect(moment.format).toHaveBeenCalledWith('HH:mm:ss');
    });

And here is the function

let result = moment(date).tz(timezone).format('HH:mm:ss');

    if (date.includes(".")) {
        result += moment(date).tz(timezone).format('.SSS');

        const fracSec = date.split(".")[1].slice(0, -1); //remove Z and get fractionnal seconds
        if (fracSec.length > 3) {
            result += `.${fracSec.slice(-(fracSec.length - 3))}`;
        }
    }
    return result;

I've tried multiple solutions, changing spyOn() function call as well as the expect(), but I never made it work. The test fail saying that the function cannot be found.

    Error: <spyOn> : format() method does not exist
    Usage: spyOn(<object>, <methodName>)

I've managed to make the spy work with this :

spyOn(moment, 'tz').and.returnValue({
            format: jasmine.createSpy()
        });


expect(moment.tz().format).toHaveBeenCalled();

but then I get an error saying that the function was never called

Error: Expected spy unknown to have been called.

If you have propositions on how I could get this spy to work, thanks in advance.