how to get the date difference i react js

i have created an date range picker to get the checkin and checkout dates and I want to get the checkin checkout date difference and update the state. after that i want to show it in the input fields. But I have no idea to get the date diffrence! if anyone knows the answer please help!

state = {
        checkin: '',
        checkout: '',
        texttwo:'

}

<DatePicker
        onChange={(e) => this.dateSelected(e, 'checkout')}
/>
<DatePicker
        onChange={(e) => this.dateSelected(e, 'checkin')}
/>
<Input
    size="default"
    placeholder="0"
    value={this.state.texttwo}
   onChange={(e) => this.calculate(e, 'two')}
 />

dateSelected = (date, dateType) => {

        if (dateType === 'checkin') {
            this.setState({
                checkin: moment(date).format('YYYY-MM-DD')
        });

        }
        else {
            this.setState({
                checkout: moment(date).format('YYYY-MM-DD')
            });
        }
        if((this.state.checkin !=='') && (this.state.checkout !=='')){
            let getCheckin = this.state.checkin;
            let getCheckout = this.state.checkout;
            let getDiff = ********
            this.setState({
                texttwo:getDiff
            });
        }

    };