:root {
  --board-bg: #1c1c1e;
  --board-face: #2c2c2e;
  --digit: #f5f5f7;
  --groove: #0a0a0a;
  --hinge: rgba(255, 255, 255, 0.06);
  --danger: #ff6b6b;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html,
body {
  height: 100%;
}

body {
  margin: 0;
  background: var(--board-bg);
  color: var(--digit);
  font-family:
    ui-sans-serif,
    system-ui,
    -apple-system,
    "Segoe UI",
    Roboto,
    "Helvetica Neue",
    Arial,
    sans-serif;
}

.pane {
  min-height: 100vh;
  min-height: 100dvh;
  display: flex;
  align-items: center;
  justify-content: center;
  padding: clamp(0.75rem, 3vw, 2rem);
}

.pane[hidden] {
  display: none;
}

.date-form {
  display: flex;
  flex-direction: column;
  gap: 1rem;
  width: min(100%, 20rem);
}

.date-form-field {
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
}

.date-form-label {
  font-size: 0.875rem;
  font-weight: 500;
  color: color-mix(in oklab, var(--digit) 88%, transparent);
}

.date-form-input {
  appearance: none;
  width: 100%;
  margin: 0;
  padding: 0.6rem 0.65rem;
  font: inherit;
  font-size: 1rem;
  line-height: 1.25;
  color: var(--digit);
  background: var(--board-face);
  border: 1px solid color-mix(in oklab, var(--digit) 18%, var(--groove));
  border-radius: 8px;
  box-shadow: inset 0 1px 0 var(--hinge);
}

.date-form-input:focus-visible {
  outline: 2px solid color-mix(in oklab, var(--digit) 45%, transparent);
  outline-offset: 2px;
}

.form-error {
  margin: 0;
  font-size: 0.8125rem;
  line-height: 1.35;
  color: var(--danger);
}

.form-error:not([hidden]) {
  margin-top: -0.25rem;
}

.form-submit {
  margin: 0;
  padding: 0.65rem 1rem;
  font: inherit;
  font-size: 0.9375rem;
  font-weight: 600;
  color: var(--board-bg);
  background: var(--digit);
  border: none;
  border-radius: 8px;
  cursor: pointer;
}

.form-submit:hover {
  filter: brightness(1.05);
}

.form-submit:focus-visible {
  outline: 2px solid color-mix(in oklab, var(--digit) 70%, transparent);
  outline-offset: 3px;
}

.change-date {
  display: block;
  margin: 0;
  padding: 0;
  color: inherit;
  background: transparent;
  border: 0;
  cursor: pointer;
}

.change-date:focus-visible {
  outline: 2px solid color-mix(in oklab, var(--digit) 70%, transparent);
  outline-offset: clamp(0.5rem, 2vw, 0.75rem);
}

.flaps {
  display: flex;
  flex-wrap: nowrap;
  gap: clamp(0.35rem, 2vw, 0.75rem);
  align-items: stretch;
}

.flap {
  --flap-w: clamp(2.75rem, 14vw, 5.5rem);
  --flap-h: clamp(4rem, 22vw, 8.5rem);
  width: var(--flap-w);
  height: var(--flap-h);
  position: relative;
  background: var(--groove);
  border-radius: clamp(4px, 1vw, 8px);
  box-shadow:
    0 0.18rem 0.45rem rgba(0, 0, 0, 0.38),
    inset 0 0 0 1px rgba(255, 255, 255, 0.05);
  overflow: hidden;
  perspective: 900px;
}

.flap::after {
  content: "";
  position: absolute;
  left: 0;
  right: 0;
  top: 50%;
  height: 2px;
  margin-top: -1px;
  background: var(--groove);
  box-shadow: 0 1px 0 var(--hinge);
  pointer-events: none;
  z-index: 1;
}

.flap-face {
  position: absolute;
  inset: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  font-family:
    ui-monospace,
    "SF Mono",
    "Cascadia Code",
    "Consolas",
    monospace;
  font-size: clamp(1.75rem, 12vw, 4.25rem);
  font-weight: 600;
  line-height: 1;
  letter-spacing: -0.02em;
  color: var(--digit);
  text-shadow:
    0 1px 0 rgba(255, 255, 255, 0.06),
    0 -1px 0 rgba(0, 0, 0, 0.75);
  user-select: none;
  backface-visibility: hidden;
  transform-style: preserve-3d;
}

.flap-top,
.flap-fold-out {
  clip-path: inset(0 0 50% 0);
  background: linear-gradient(
    180deg,
    color-mix(in oklab, var(--board-face) 86%, #fff) 0%,
    var(--board-face) 46%,
    color-mix(in oklab, var(--board-face) 78%, #000) 50%
  );
}

.flap-bottom,
.flap-fold-in {
  clip-path: inset(50% 0 0 0);
  background: linear-gradient(
    180deg,
    color-mix(in oklab, var(--board-face) 74%, #000) 50%,
    var(--board-face) 56%,
    color-mix(in oklab, var(--board-face) 68%, #000) 100%
  );
}

.flap-top,
.flap-bottom {
  z-index: 0;
}

.flap-fold-out,
.flap-fold-in {
  opacity: 0;
  z-index: 2;
}

.flap-fold-out {
  transform-origin: 50% 50%;
}

.flap-fold-in {
  transform: rotateX(90deg);
  transform-origin: 50% 50%;
}

.flap.is-flipping .flap-fold-out {
  opacity: 1;
  animation: fold-out 0.42s cubic-bezier(0.2, 0.72, 0.28, 1) both;
}

.flap.is-flipping .flap-fold-in {
  opacity: 1;
  animation: fold-in 0.42s cubic-bezier(0.2, 0.72, 0.28, 1) 0.18s both;
}

.flap.is-flipping::before {
  content: "";
  position: absolute;
  inset: 50% 0 0;
  background: linear-gradient(180deg, rgba(0, 0, 0, 0.55), transparent 58%);
  opacity: 0;
  pointer-events: none;
  z-index: 3;
  animation: hinge-shadow 0.6s ease-out both;
}

@keyframes fold-out {
  0% {
    transform: rotateX(0deg);
    filter: brightness(1);
  }
  48% {
    filter: brightness(0.72);
  }
  100% {
    transform: rotateX(-92deg);
    filter: brightness(0.42);
  }
}

@keyframes fold-in {
  0% {
    transform: rotateX(92deg);
    filter: brightness(0.45);
  }
  45% {
    filter: brightness(0.78);
  }
  100% {
    transform: rotateX(0deg);
    filter: brightness(1);
  }
}

@keyframes hinge-shadow {
  0% {
    opacity: 0;
  }
  48% {
    opacity: 0.72;
  }
  100% {
    opacity: 0;
  }
}

@media (prefers-reduced-motion: reduce) {
  .flap.is-flipping .flap-fold-out,
  .flap.is-flipping .flap-fold-in,
  .flap.is-flipping::before {
    animation: none;
  }
}