About Atlas Time
Atlas Time keeps the world in view. Daylight, darkness, date and time change together with a few simple gestures
Inside Atlas Time
One shared moment.
Move the clock forward or backward to compare local time, date, and daylight across cities at the same moment. The map and city clocks update together, providing a visual reference to help coordinate a business meeting or find the right time to call a loved one while traveling.
Offline maps
Earth imagery.
Atlas Time includes media published by NASA Earth Observatory.* The app cycles through the seasonal daylight images and reveals the nighttime image on the dark side of the calculated day-night terminator. The terminator is calculated separately and is not part of the source images.
Image credits: NASA Earth Observatory.* Blue Marble Next Generation (2005) by Reto Stöckli et al.; Black Marble 2016 by Joshua Stevens, using Suomi NPP VIIRS data from Miguel Román, NASA GSFC.
*NASA does not endorse Atlas Time, and no affiliation is implied.
Calculated daylight
The line between day and night
The subsolar point is the location on Earth where the Sun is directly overhead at a specific date and time. Atlas Time calculates this point for the selected moment. Its latitude is the solar declination, which shifts north and south over the course of the year. Its longitude is calculated from Coordinated Universal Time (UTC), with an adjustment for the difference between apparent solar time and mean solar time, and moves west as Earth rotates.
Using the subsolar point, Atlas Time evaluates locations across the map to determine whether the Sun’s geometric center is above or below the local horizon. Locations with the Sun above the horizon are shown in daylight; locations with it below the horizon reveal the nighttime imagery.
The day–night terminator is formed by the points where the Sun’s center is on the geometric horizon. One side of the boundary marks geometric sunrise, while the other marks geometric sunset. On a globe, the terminator forms a circle. On a rectangular map projection, it appears as a curve. The selected time moves this curve east or west across the map, while Earth’s axial tilt changes its shape throughout the year.
How Atlas Time classifies each point
For a point at latitude φ, the Sun’s geometric altitude h is determined by solar declination δ and solar hour angle H:
sin(h) = (sin(φ) sin(δ)) + (cos(φ) cos(δ) cos(H))
Where time appears
The equation does not contain a separate time variable because the selected date and time have already been incorporated into δ and H. The selected date determines the Sun’s seasonal position through δ. The selected UTC time, each point’s longitude, and the equation of time determine H, which changes continuously as Earth rotates.
How the result is used
- h > 0°
- The Sun is above the horizon: daylight.
- h = 0°
- The Sun’s center is on the geometric horizon: the day–night terminator.
- h < 0°
- The Sun is below the horizon: darkness.
The soft edge around the boundary is a visual treatment centered on the calculated terminator. It does not represent a measured physical twilight band.
Sunrise and sunset times on city cards are calculated separately using the apparent horizon, including standard approximations for atmospheric refraction and the apparent radius of the Sun’s disk. After sunset, the card continues to display the sunset time through civil twilight, which ends when the Sun’s center reaches 6° below the horizon.
Civil time
A place is more than an offset.
Civil time is the local time used in everyday life, set by governments through time-zone and daylight-saving rules. It is shaped by policy as much as geography. A place’s offset from UTC can vary by season, and its rules can change over time or with little notice.
Atlas Time identifies each city with a time-zone identifier from the Internet Assigned Numbers Authority (IANA) Time Zone Database (for example, America/Chicago or Asia/Kathmandu) rather than a fixed offset from UTC. For the exact instant shown on the map, the app calculates the applicable UTC offset, zone abbreviation, local date, and daylight-saving state.
Testing the edges of civil time.
Tests cover skipped and repeated local times, southern-hemisphere seasons, UTC offsets in 30- and 45-minute increments, Lord Howe Island’s 30-minute daylight-saving change, Morocco’s 2026 Ramadan adjustment, and historical rule changes.
Kiritimati (UTC+14) and Pago Pago (UTC−11) make the distinction especially visible. At the same instant, their displayed local times are 25 hours apart: their clock times differ by one hour, while the International Date Line places them on different calendar dates. Yet the two places are only about 13° apart in longitude, so they experience sunrise roughly an hour apart in elapsed time, with some seasonal variation. The 25-hour difference comes from civil time, not a comparable physical distance around Earth.
Calculation boundary
Atlas Time performs these calculations on device using the time-zone rules installed with iOS. The bundled city catalog identifies each place; iOS evaluates the civil-time rules that apply at the selected instant. Because governments can revise those rules, future results depend on the time-zone data available on the device.
Offline database
The city catalog.
City search does not call a web service. Atlas Time carries a curated catalog of 6,650 searchable cities and 312 representative IANA time zones. It includes coordinates, alternate names, and country and region information used to identify each place and position its beacon.
No single source contains all of this information. IANA supplies canonical time-zone identities; the Unicode Common Locale Data Repository (CLDR) supplies familiar time-zone names and aliases; GeoNames supplies places and alternate names; and Natural Earth supplies capital status, geographic importance, and city coordinates. Retrieval dates are recorded for every input, and each source is used in accordance with its license in the app’s database.
The curated data set
An early import produced 34,117 records, including neighborhoods and localities that made city search noisier rather than more useful. Atlas Time’s curation algorithm retains capitals, major cities, recognized Natural Earth places, and at least one representative for every canonical geographic time zone. The resulting catalog contains 6,650 cities while preserving all 312 representative zones.
Optimized search
Geographic importance, capital status, and population all factor into search ranking alongside exact-name matches, so Chicago appears before Chicago Lawn and London in the United Kingdom before London, Ontario. Search also recognizes alternate names, IANA identifiers, and familiar time-zone terms, and works with or without accent marks (for example, São Paulo or Sao Paulo).
Available now
Keep the world in view.
Download Atlas Time for iPhone and iPad.
