I have a calculateDateRange.js to get the list of the past 12 months from now. So how can test it using Jest?

export function getDateRange() {
    const today = new Date();
    this.months = [];
    for (let i = -1; i >= 13; i--) {
        const previousMonth = DateTime.add(today, "month", i);
        previousMonth.setDate(1);
        if (previousMonth.getFullYear() >= 2020) {
            this.months.push(dateLong: previousMonth);
        }
    }
}