Pushed objects have different property values before and after push

Question

I have a problem with creating array of objects in a while loop. I create an object newDay with date starting from today and slotsId object, which is a mongo Id object. I am trying to create array of custom Day objects using while loop. In each itaration I'm creating newDay and then I push it into daysArray and then increase . When I log each day before performing push, it logs as intended - each day has unique date. But after finishing the loop, all days in array have the same date - date of last pushed day. I really have no clue what is going on. What am I missing?

Day object:

class Day implements ISingleDay {
    date: Date;
    workday: boolean;
    slots: mongoose.Types.ObjectId | null;
    _id: mongoose.Types.ObjectId

    validateHoliday() {
        const dateDay = String(this.date.getDate()).padStart(2, '0');
        const dateMonth = String(this.date.getMonth() + 1).padStart(2, '0');
        const dateYear = String(this.date.getFullYear());
        const monthAndDay = dateMonth + '-' + dateDay;
        const currentDate = new Date(dateYear + '-' + monthAndDay);
        if (currentDate.getDay() === 0 || currentDate.getDay() === 6 || holidays.includes(monthAndDay)) {
            return false;
        } else {
            return true;
        }
    }
    constructor(date: Date, slots: mongoose.Types.ObjectId) {
        this.date = date;
        this.workday = this.validateHoliday();
        this._id = new mongoose.Types.ObjectId()
        if (this.validateHoliday()) {
            this.slots = slots;
        } else {
            this.slots = null;
        }
        console.log(this.date)
    }
}

Note: method validateHoliday() in this form is temporary.

Function I'm talking about:

export function createDayArray(doctorId: mongoose.Types.ObjectId, daysId: mongoose.Types.ObjectId, firstname: string, lastname: string) {
    const slotsArray = createSlotArray();
    const dayCount = workdaySettings.days.dayCount || 30;
    const daysArray: Day[] = [];
    const todayNonUTC = new Date();
    const date = new Date(Date.UTC(todayNonUTC.getUTCFullYear(), todayNonUTC.getUTCMonth(), todayNonUTC.getUTCDate(), 0, 0, 0, 0));

    while (daysArray.length <= dayCount) {
        const dayId = new mongoose.Types.ObjectId();
        const slotsId = new mongoose.Types.ObjectId();
        const newDay = new Day(date, slotsId);
        daysArray.push(newDay);
        date.setDate(date.getDate() + 1);
        new Slots({
            _id: slotsId,
            doctorId: doctorId,
            dayId: dayId,
            slots: slotsArray
        }).save()
    }
    new Days({
        _id: daysId,
        doctorId: doctorId,
        doctorName: `${firstname} ${lastname}`,
        days: daysArray
    }).save()
}