<div class="inputs-wrapper">
        <div class="cooking">
            <p>Время готовки:</p>
        <input class="hour cooking-hours" type="text" name="clock_hour" value="0">
        <input class="minut cooking-mins" type="text" name="clock_minut" value="0">
        </div>
        <div class="prep">
            <p>Время подготовки:</p>
            <input class="hour prep-hours" type="text" name="clock_hour1" value="0">
            <input class="minut prep-mins" type="text" name="clock_minut1" value="0">
        </div>
    </div>
    <div class="output-wrapper">
        <p>
            Общее время: 
            <span class="total-hours">00</span>:
            <span class="total-mins">00</span>
        </p>
    </div>

    <script>
        document.addEventListener('DOMContentLoaded', function(){
            let inputs = document.getElementsByTagName('input');
            for(let i = 0; i < inputs.length; i++){
                let input = inputs[i];
                input.addEventListener('change', function(e){
                    let timeInSeconds = valuesToSeconds();
                    sec2time(timeInSeconds);
                });
            }
        });

        function valuesToSeconds(){
            let totalSeconds = document.getElementsByClassName("cooking-hours")[0].value * 3600 +
                document.getElementsByClassName("cooking-mins")[0].value  * 60 + 
                document.getElementsByClassName("prep-hours")[0].value * 3600 + 
                document.getElementsByClassName("prep-mins")[0].value   * 60;
            return totalSeconds;
        }

        function sec2time(timeInSeconds) {
            var pad = function(num, size) { return ('000' + num).slice(size * -1); },
            time = parseFloat(timeInSeconds).toFixed(3),
            hours = Math.floor(time / 60 / 60),
            minutes = Math.floor(time / 60) % 60,
            seconds = Math.floor(time - minutes * 60),
            milliseconds = time.slice(-3);
            setOutput(pad(hours, 2), pad(minutes, 2));
        }

        function setOutput(hours, minutes) {
            document.getElementsByClassName("total-hours")[0].innerText = hours;
            document.getElementsByClassName("total-mins")[0].innerText = minutes;
        }
    </script>