Longitude & Time Calculator
Find the local time at any longitude with full working, see why IST is GMT + 5:30, compare live clocks of 40+ world cities and watch day and night move across the Earth.
The Earth turns 360° in 24 hours, so 15° of longitude equals 1 hour and 1° equals 4 minutes. India's Standard Meridian is 82°30′ E, so 82.5 × 4 = 330 minutes = 5 hours 30 minutes; because it lies east of Greenwich, IST = GMT + 5:30. Going east you add time, going west you subtract.
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Step-by-step working
Day and night across the longitudes
The sunlit half of the Earth (along the Equator) for the time shown. Lines every 15° = 1 hour.
Showing the live time. Drag the slider to move up to 12 hours forward or back.
World cities compared with IST
Clock time follows each country’s time zone (including daylight saving); sun time follows the longitude.
| City | Longitude | Clock time now | vs IST | Time zone | Sun time* | Sun |
|---|
* Local mean time from the longitude: UTC + longitude × 4 minutes. It differs from clock time because whole countries share one standard time.
India’s Standard Meridian
India stretches about 30° from west to east, so the Sun rises nearly 2 hours earlier in Arunachal Pradesh than in Gujarat.
Why IST is GMT + 5:30
- The Standard Meridian of India is 82°30′ E = 82.5° east of Greenwich.
- Time difference = 82.5 × 4 min = 330 minutes.
- 330 min = 5 hours 30 minutes.
- It is east of Greenwich, so India is ahead: IST = GMT + 5:30.
- When it is 12:00 noon at Greenwich, it is 5:30 PM in India.
Sun time in Indian cities when the clock shows 12:00 noon IST
Everyone in India uses IST, but the Sun is “ahead” in the east and “behind” in the west.
| Place | Longitude | Difference from 82°30′E | Local (sun) time |
|---|
Quick method
- Find the difference in longitude — subtract if both are E (or both W), add if one is E and one is W.
- Multiply by 4 minutes per degree (4 seconds per minute of arc).
- Going east → add time; going west → subtract time.
- East Greenwich Gains, West Greenwich Loses (“EGG-WGL”).
10 mixed questions on GMT, IST, longitude and the Date Line. Keys 1–4 to answer, Enter for next.
From longitude to local time
The Earth turns from west to east, so places to the east see the Sun first and their clocks are ahead.
Enter place A
Type its longitude in degrees and minutes with E or W, and the time you know there.
Enter place B
Type the second longitude, or pick a city from the list.
Read the working
See the difference in longitude, the time difference and whether to add or subtract.
Explore & practise
Compare live world clocks, see day and night, and try the practice questions.
Local time formula
Time diff = Δ longitude × 4 min
Example: Delhi (77°E) and London (0°): 77 × 4 = 308 min = 5 h 8 min. Delhi’s local (sun) time is 5 h 8 min ahead of London’s.
Local time vs standard time
Local time depends on a place’s own longitude — it is noon when the Sun is highest overhead. Standard time is one agreed time for a whole country or zone, based on a central (standard) meridian, so that trains and offices use the same clock.
International Date Line
The Date Line roughly follows 180° and bends around islands. Crossing it westward (from America to Asia) you move the calendar one day forward; crossing eastward you go one day back. Kiribati’s Line Islands are at UTC + 14, among the first places to start each new day.
Longitude & Time Calculator — your questions
Why is Indian Standard Time GMT plus 5:30?
India's Standard Meridian is 82°30′ E, which is 82.5° east of Greenwich. Since 1° equals 4 minutes of time, 82.5 × 4 = 330 minutes, and 330 minutes is 5 hours 30 minutes. Because the meridian lies east of Greenwich, India is ahead, so IST = GMT + 5:30.
Why is 15 degrees of longitude equal to 1 hour?
The Earth completes one full rotation of 360° in 24 hours, so in one hour it turns 360 ÷ 24 = 15°. That makes 1° equal to 60 ÷ 15 = 4 minutes of time, and one minute of arc equal to 4 seconds. These three figures solve almost every longitude and time question.
How do I calculate local time from longitude?
Find the difference in longitude — subtract if both places are east or both west, add if one is east and one west — then multiply by 4 minutes per degree. Add the result if you are going east and subtract if going west. Delhi at 77° E is 77 × 4 = 308 minutes, or 5 hours 8 minutes, ahead of Greenwich.
What is the difference between local time and standard time?
Local time depends on a place's own longitude — it is noon there when the Sun is highest overhead, so every town has a slightly different local time. Standard time is one agreed clock time for a whole country or zone, based on a chosen standard meridian, so that trains, schools and offices can share a timetable.
Why does India have only one time zone?
India uses a single standard meridian at 82°30′ E for the whole country. Because India stretches about 30° from west to east, the Sun rises nearly two hours earlier in Arunachal Pradesh than in Gujarat: 97°25′ E minus 68°7′ E is 29°18′, which is about 117 minutes of sun time. This is why a second time zone is sometimes suggested.
What happens when you cross the International Date Line?
The Date Line roughly follows 180° longitude, bending around island groups. Crossing it westward, from America towards Asia, you move the calendar one day forward; crossing eastward you go one day back. Kiribati's Line Islands sit at UTC + 14 and are among the first places on Earth to begin each new day.
Why are standard meridians often multiples of 7 degrees 30 minutes?
So that a country's time differs from GMT in neat steps of half an hour. 7°30′ of longitude is exactly 30 minutes of time, and India's 82°30′ is 11 × 7°30′, giving the half-hour offset of 5:30. Most countries choose a multiple of 15° instead, giving a whole number of hours.