style.css

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

:root {
  font-size: 62.5%; /* 10px/16px = 62.5% -> 1rem = 10px */
  font-family: sans-serif;
}

body {
  background: #333;
  color: #eee;
  min-height: 100vh;
  display: grid;
  grid-template-rows: 1fr min-content;
  align-items: center;
  justify-content: center;
}

.timeDateContainer p {
  padding: 0.5rem;
  font-size: 6rem;
  font-variant-numeric: tabular-nums;
  text-align: center;
}

.btn {
  width: 100vw;
  font-family: inherit;
  font-size: 1.5rem;
  cursor: pointer;
  padding: 25px 80px;
  display: inline-block;
  color: #333;
  background: #eee;
  border: 4px solid #eee;
  text-transform: uppercase;
  letter-spacing: 1px;
  font-weight: 700;
  outline: none;
  position: relative;
  transition: all 0.3s;
}

.btn:hover {
  background: transparent;
  color: #eee;
}