UPSSSC PET · Bite-size lessons
Time Zones
About 7 min · Geography · English
Time worked out from the lesson's 488 words at 150 words a minute, plus a minute for each question.
Time zones are a fundamental concept in geography that appear in UPSSSC PET exams. Understanding how Earth's rotation creates different local times across longitudes is essential for answering questions about India Standard Time (IST), Greenwich Mean Time (GMT), and international time calculations.
Key ideas
- Earth's rotation and time: Earth rotates 360° in 24 hours, meaning 15° of longitude equals 1 hour (360 ÷ 24 = 15). This is the foundation of all time-zone calculations.
- Local time vs Standard time: Local time varies continuously with longitude; standard time is uniform across a country or region, based on a chosen standard meridian to avoid confusion.
- Prime Meridian (0° longitude): The reference line passing through Greenwich, London. Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC) is measured from here.
- India Standard Time (IST): India follows a single time zone based on 82.5°E longitude, which passes near Mirzapur (Uttar Pradesh). IST is GMT + 5:30 hours.
- East-West rule: Places east of a reference point experience sunrise earlier, so their local time is ahead. Places west see sunrise later, so their time is behind.
Watch out for
- Forgetting the direction rule: Students often add time when moving west or subtract when moving east. Remember: "East gain, West lose" — moving eastward adds time, westward subtracts.
- Confusing longitude with time zones: Not all time zones are exactly 15° wide. Political boundaries often adjust them. Always use the standard meridian of a country/region, not just its physical longitude.
- Mixing up GMT and IST offsets: Some students subtract 5:30 from GMT to get IST or add it incorrectly. IST is always ahead of GMT, so add 5:30 to GMT time to get IST.
Quick reference
- Earth rotation: 360° in 24 hours → 15°/hour → 1°/4 minutes
- India Standard Meridian: 82.5°E (passes through Mirzapur, UP)
- IST formula: GMT + 5 hours 30 minutes
- Time calculation: Longitude difference ÷ 15 = time difference in hours
- Direction mnemonic: East gain, West lose (add time moving east, subtract moving west)
Check yourself: 3 questions
1. India's Standard Time is based on the 82.5° E longitude which passes through Mirzapur near Allahabad in Uttar Pradesh. If it is 12:00 noon IST in India, what will be the time at Greenwich (0° longitude) at that moment?
2. A city is located at 75° W longitude. Another city is located at 105° E longitude. If it is 3:00 PM at the city located at 75° W, what will be the time at the city located at 105° E?
3. An international flight departs from Tokyo (139° 45' E) at 10:00 AM local time on Monday and flies to San Francisco (122° 25' W). The flight duration is 10 hours. Approximately what will be the local time and day when the plane lands in San Francisco? (Assume no daylight saving adjustments)
This lesson is cut from the topic's note, not rewritten. Fact-checked on 17 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain. The questions are from Shishya's validated question bank.
Read the full noteFlashcards for this topic
Saves this lesson and its 3 questions in this browser on this phone. No sign-in; nothing about you is saved or sent. A saved copy is not updated until you save it again. Parts that load from the server when you use them (the AI tutor, adaptive practice, sign-in) still need a connection.