Timer JS with words ending

TimerJS

function decOfNum (number, titles) {

	if(!decCache[number]) decCache[number] = number % 100 > 4 && number < 20 ? 2 : decCases[Math.min(number % 10, 5)];
	return titles[decCache[number]];
}

function timerLeft() {
	setInterval(function() {
		var hoursLeft = document.getElementById('timer-v-hours'),
			hoursText = document.getElementById('timer-v-hours_text'),
			minutesLeft = document.getElementById('timer-v-minutes'),
			minutesText = document.getElementById('timer-v-minutes_text'),
			secLeft = document.getElementById('timer-v-seconds'),
			secText = document.getElementById('timer-v-seconds_text'),

			dateNow = new Date().getTime(),
			dateFinish = new Date(2018, 12, 31),

			sec = Math.floor((dateFinish - dateNow) / 1000),
			secToMinutes = sec % 3600,
			hoursNumber = Math.floor(sec / 3600),
			minnutesNumber = Math.floor(secToMinutes / 60),
			secondsNumber = secToMinutes % 60;

		hoursText.innerHTML = decOfNum(hoursNumber, ['час', 'часа', 'часов']);
		minutesText.innerHTML = decOfNum(minnutesNumber, ['минута', 'минуты', 'минут']);
		secText.innerHTML = decOfNum(secondsNumber, ['секунда', 'секунды', 'секунд']);
		hoursLeft.innerHTML = hoursNumber;
		minutesLeft.innerHTML = minnutesNumber;
		secLeft.innerHTML = secondsNumber;
	})
}

timerLeft();