wp-calypso/client/components/date-picker/day.jsx

const noop = () => {};

const handleDayMouseEnter =
	( date, modifiers, onMouseEnter = noop ) =>
	( event ) => {
		onMouseEnter( date, modifiers, event );
	};

const handleDayMouseLeave =
	( date, modifiers, onMouseLeave = noop ) =>
	( event ) => {
		onMouseLeave( date, modifiers, event );
	};

const DatePickerDay = ( { date, modifiers, onMouseEnter, onMouseLeave } ) => {
	return (
		<div
			className="date-picker__day"
			onMouseEnter={ handleDayMouseEnter( date, modifiers, onMouseEnter ) }
			onMouseLeave={ handleDayMouseLeave( date, modifiers, onMouseLeave ) }
		>
			{ date.getDate() }
		</div>
	);
};

export default DatePickerDay;