How to use the react-day-picker.DateUtils.isSameDay function in react-day-picker

disabledDays={day => {
                if (!laterThan) return
                const beforeDate = lazyDate(laterThan)
                return day < beforeDate || DateUtils.isSameDay(day, beforeDate)
              }}
              ref={el => {

              selectedDays={day => DateUtils.isSameDay(this.state.selectedDay, day)}
              localeUtils={this.localeUtils}

      modifiers: { selected: date => DateUtils.isSameDay(value, date) },
      onDayClick: locals.onSelect,

              selectedDays={day => DateUtils.isSameDay(state.day, day)}
              disabledDays={day => {

                selectedDays={ day => DateUtils.isSameDay(value, day) }/>

                        selectedDays={(day) => DateUtils.isSameDay(pickedDay, day)}
                        onDayClick={this.handleDatePick.bind(this)}

const selectedIndex = selectedDays.findIndex(selectedDay =>
        DateUtils.isSameDay(selectedDay, day)
      )

const selectedIndex = selectedDays.findIndex(selectedDay =>
        DateUtils.isSameDay(selectedDay, day)
      );