geo-clock-card

A Home Assistant Lovelace card that turns one of your dashboards into a live world clock, modeled on the Geochron® style (no affiliation). NASA Blue Marble (day) and Black Marble (night) imagery, a real day/night terminator computed from solar geometry, and DST-aware hover popups for every IANA time zone on the planet.

🌍 Live demo: https://geoclock.world

🧩 Chrome new-tab extension: a standalone, fully-offline build of the same card that replaces your new-tab page — see chrome-extension/.

What it does

• World map — NASA Blue Marble + Black Marble imagery, with the live day/night boundary recomputed from the subsolar point. 24 daylight composites (start + mid of each month, sampled from NASA SVS daily frames) swap automatically through the year so vegetation and snow cover follow the seasons.
• Twilight glow — warm sunrise/sunset rim along the terminator, screen- blended over the day side.
• Hour band — 25 column meridians at every 15° of longitude, with the current local hour highlighted at noon and midnight.
• Time-zone overlay:
  • 25 visible 15° offset bands across the globe.
  • 419 IANA polygons sitting invisibly on top — hover (or tap on mobile) and a popup shows DST-aware live time, the long zone name (EDT vs EST auto-switches), the current offset, the IANA tzid, the local date (handy when the remote zone has rolled to a different day), and the hovered region outlines itself for visual feedback.
  • Time and date follow the browser's locale (24-hour vs AM/PM, weekday/month language).
  • Open ocean / Antarctic strips with no IANA polygon fall through to the offset band's popup.
• Centering modes — sun (default, subsolar point — the daylit hemisphere stays in the middle and the map slowly drifts), home (your HA-configured longitude), longitude (any numeric value), or entity (follow a zone / person / device-tracker entity). All non-sun modes fall back to Greenwich (lon=0°) with a console warning when their data isn't available — so a misconfigured mode looks visibly different from sun instead of silently behaving the same.
• Home marker — optional dot at your HA-configured location that tracks the map's drift, with an opt-in label + current local time beneath it.
• Location markers — pin any list of HA entities (zones, persons, device-trackers) on the map; each shows the entity's label and the current local time at that zone (DST-aware, looked up from its longitude/latitude). Labels are always-visible by default; flip to hover-only if you'd rather keep the map tidy. The weekday is appended to the time so a marker on the far side of the planet whose date has rolled over is obvious at a glance — toggle off if you want time-only. Per-marker color, or a card-wide default that themes can override via --geo-marker-color.
• Main-clock time source — the wall-clock readout can read from your HA location (default), the viewing device, or any HA entity with a longitude/latitude. (Breaking change in v0.2.0: pre-0.2.0 cards behaved as if the source was the viewing device. Set mainTimeSource: device to keep the old behavior.)
• Time scrubbing — freeze the clock at any UTC moment via now: … for screenshots or to preview the look at, say, the December solstice.
• Visual editor — opens automatically when adding the card; advanced visual knobs collapse into an expansion panel so the form stays clean.
• Tunable — twilight band size, glow color/opacity, day brightness, night contrast, time-zone line color, all via card config or CSS variables.
• Cheap — clock readout ticks every second; the map only recomputes when the subsolar point would have moved ≥ 0.5 px on a 4K display (≈ every 10.5 s). When the card is off-screen or the browser tab is backgrounded the timer drops to a 30-minute cadence; refreshes immediately when the card becomes visible again.

Install

HACS (recommended)

1. Click the badge above (it deep-links to your HA instance) — or manually go HACS → ⋮ → Custom repositories → add https://github.com/jpettitt/geo-clock-card, category Lovelace.

2. Install Geo Clock Card from HACS.

3. Refresh the browser. HACS adds the resource for you; otherwise: Settings → Dashboards → ⋮ → Resources → add /hacsfiles/geo-clock-card/geo-clock-card.js as a JavaScript Module.

4. Add a card to your dashboard:

   type: custom:geo-clock-card

Manual

1. Clone the repo and build:

   git clone https://github.com/jpettitt/geo-clock-card
   cd geo-clock-card
   npm install
   npm run fetch-assets   # downloads NASA imagery + IANA TZ polygons
   npm run build

2. Copy the entire dist/ directory into your Home Assistant config:

   <config>/www/community/geo-clock-card/
       geo-clock-card.js
       blue-marble-*-2048.jpg
       black-marble-2048.jpg
       timezones.json
       timezones-iana.json
   

3. Add the resource: Settings → Dashboards → ⋮ → Resources → /local/community/geo-clock-card/geo-clock-card.js, type JavaScript Module.

4. Add the card with type: custom:geo-clock-card.

Configuration

A visual editor opens automatically when you add the card from the Lovelace picker. Every option is also settable in YAML:

type: custom:geo-clock-card

# Map centering — sun (default) | home | longitude | entity
center: sun
centerLongitude: -119       # required when center: longitude. -180..180.
centerEntity: zone.home     # required when center: entity. Must expose a longitude attribute.
showHomeMarker: false       # render a dot at hass.config.latitude/longitude
showHomeMarkerLabel: false  # also show home name + current local time under the dot

# Imagery + atmosphere (under "Advanced visual settings" in the editor)
dayBrightness: 1.15         # CSS brightness() on the day layer (0.5..2.0)
nightContrast: 1.0          # CSS contrast() on the night layer (0.5..3.0)
twilightDegrees: 8          # half-band of the day/night fade, sun-elevation degrees (1..18)
twilightColor: "#463701"    # hex / rgb / rgba / hsl / named color
twilightOpacity: 0.26       # 0..1
timezoneLineColor: "rgba(255, 255, 255, 0.18)"  # any CSS color string

# Time
updateInterval: 1           # seconds between clock-readout ticks (1..600).
                            # The map repaints on a separate auto-throttled timer;
                            # both drop to 30 minutes when the card is off-screen.
now: "2024-12-21T09:21:00Z" # freeze the clock at this moment (omit for live)

# Main-clock time source — home (default) | device | entity
# - 'home'   reads hass.config.time_zone (the HA-configured zone)
# - 'device' uses the viewing browser's zone (pre-0.2.0 behavior)
# - 'entity' picks the IANA zone of an HA entity's lat/lon
mainTimeSource: home
mainTimeEntity: zone.work   # required when mainTimeSource: entity

# Location markers — list of HA entities to pin on the map.
# Each entity must expose numeric `latitude` and `longitude` attributes.
markers:
  - entity: zone.work
    label: Office            # optional — defaults to the entity's friendly_name
    color: "#3da9fc"         # optional — defaults to markerColor
  - entity: person.alice
markerLabelMode: always     # always | hover
markerColor: "#3da9fc"      # default fill for markers without their own color
                            # — omit (or unset) to let `--geo-marker-color` win
markerShowDay: true         # append weekday after the time (e.g. "12:22 PM Friday")

# Overlays
showTimezoneBand: true      # hour-of-day numbers across the top
showTimezoneBoundaries: true # IANA hover/identify popup hit layer
showTimezoneRegions: true   # 15° offset rectangle bands (defaults to
                            # showTimezoneBoundaries; set independently to
                            # show the bands as chrome without the popup)
showTimezonePopup: true     # live-time popup on hover/tap
showUTC: true               # UTC time below the local clock

# Localization (optional; defaults to the browser/runtime locale)
locale: ""                  # BCP-47 tag, e.g. "fr-FR" or "ja-JP".
                            # Governs the popup's localized timezone
                            # name, the clock readout, marker times, and
                            # the 12/24-hour choice. Empty = follow the
                            # viewer's browser language.

# Bundle resolution (rarely needed; not exposed in the visual editor)
imageryBase: ""             # override the assets URL (e.g. host on a CDN)

Centering modes

• sun (default) — centered on the current subsolar longitude. The map drifts westward as the sun moves; the daylit hemisphere stays in the middle.
• home — centered on hass.config.longitude. Falls back to Greenwich (0°) with a console.warn if HA hasn't reported a longitude.
• longitude — centered on a numeric centerLongitude value. Falls back to Greenwich (0°) if the value is missing or non-numeric.
• entity — centered on the longitude attribute of an HA entity. Works with zone.*, person.*, and most device_tracker.* entities. Falls back to Greenwich (0°) if the entity is missing or has no numeric longitude.

The Greenwich fallback is deliberate — it's visually distinct from sun mode so a misconfiguration doesn't silently look right. Watch the browser console for a one-off warn line that names the broken field.

Theme via CSS variables

The card respects HA dashboard themes via custom-element CSS variables. Drop these in your theme YAML (or use card-mod) to override without touching the card config:

geo-clock-card:
  --geo-day-brightness: 1.2
  --geo-night-contrast: 1.0
  --geo-twilight-color: "#5a4a1a"
  --geo-twilight-opacity: 0.3
  --geo-tz-bg: rgba(8, 14, 28, 0.85)
  --geo-tz-noon: "#ffd866"
  --geo-tz-mid: "#6ab0ff"
  --geo-tz-line: rgba(255, 255, 255, 0.18)
  --geo-home-marker: var(--accent-color, "#ff7a3d")
  --geo-marker-color: "#3da9fc"

Most of these have a matching named config option (e.g. dayBrightness → --geo-day-brightness); set in YAML when you're tweaking a single card, or in a theme when you want every dashboard's geo-clock to match.

Imagery sources (all public domain)

Layer	Source	Resolution
Day (Blue Marble)	NASA SVS dataset 3523, daily frames interpolating monthly Blue Marble Next Generation composites	2048 × 1024 JPEG, 24 frames (start + mid of each month)
Night (Black Marble)	NASA "Earth at Night" 2012 composite	2048 × 1024 JPEG
Time-zone offsets	Generated as 25 × 15° rectangle bands	8 KB GeoJSON
Time-zone IANA	timezone-boundary-builder 2026b, simplified to 1% retention	~960 KB GeoJSON

Develop

npm install
npm run fetch-assets    # one-time: pulls + resizes imagery, downloads + simplifies TZ data
npm test                # vitest — solar math, projection, polygon, IANA helpers
npm run build           # rollup → dist/geo-clock-card.js + copies assets
npm run dev             # rollup --watch

A standalone preview lives at dev/index.html. Run a static server from the repo root (python3 -m http.server 8765) and open http://localhost:8765/dev/index.html. The preview gives you sliders for twilight band, day brightness, night contrast, twilight color/opacity, center mode, time-scrubbing with solstice/equinox presets, etc.

How the pieces fit together

• src/sun.ts — subsolar point + sun-elevation math (USNO low-precision formulas).
• src/terminator.ts — terminator polygon and curve, parameterized by center longitude.
• src/projection.ts — equirectangular projection with arbitrary center.
• src/timezones.ts — offset overlay (25 rectangle bands + popup data).
• src/timezones-iana.ts — IANA polygon hit-testing layer + DST-aware Intl.DateTimeFormat wrapper.
• src/timezone-band.ts — top-of-map hour band + tick lines.
• src/day-image.ts — picks the right monthly daylight image for a given date.
• src/geo-clock-card.ts — the Lit element that wires it all together.
• src/geo-clock-card-editor.ts — visual config editor (HA ha-textfield / ha-switch / ha-formfield / ha-selector / ha-expansion-panel, plus native <select> for the centering / time-source / marker-mode dropdowns where ha-select was unreliable). Awaits loadCardHelpers() before rendering so the entity selectors register correctly. Lazy-loaded by the card via getConfigElement() and bundled inline.

The original design notes are in DESIGN.md.

License

MIT for the code. NASA imagery is public domain. timezone-boundary-builder data is ODbL.

Geochron® is a registered trademark of Geochron Enterprises Inc.; this project is not affiliated with or endorsed by them.