#clock {
    font-size: 50px;
    width: 300px;
    margin-left: 30px;
    margin-right: 30px;
    text-align: center;
    border: 2px solid black;
    border-radius: 20px;
}

section #clock {
    display: block;
    margin-left: auto;
    margin-right: auto;
}