GeographyCBSE Class 11 · Latitude, Longitude and Time

Longitude and Time: Standard Time, Time Zones and the International Date Line (Class 11 Practical Work in Geography, Chapter 3)

Updated 17 October 2026 · Fact-checked

The earth turns 360 degrees in 24 hours, so every 15 degrees of longitude makes 1 hour and every degree makes 4 minutes. Places east are ahead and places west are behind Greenwich. A country uses a standard meridian (IST: 82 degrees 30 minutes E, GMT plus 5 h 30 min); the world has 24 time zones, and the International Date Line near 180 degrees changes the date.

Understand Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line

Why this topic matters. Time-difference sums and the date line rules are the highest-yield part of this practical chapter, and each sum can be checked by the 4-minutes-per-degree rule.

1. Basis. The earth rotates west to east, so the sun appears to rise in the east. One rotation, 360 degrees, takes 24 hours: 15 degrees of longitude per hour and 1 degree every 4 minutes.

2. Rule. Places east of a meridian are ahead in time (add 4 minutes per degree); places west are behind (subtract 4 minutes per degree). NCERT examples: when Greenwich is at 12 noon, Thimphu (90 degrees E) is 90 x 4 = 360 min = 6 hours ahead, i.e. 6 pm; New Orleans (90 degrees W) is 6 hours behind, i.e. 6 am.

3. Steps for any sum. (i) Find the longitude difference (subtract if both are on the same side of 0 degrees, add if on opposite sides). (ii) Multiply by 4 minutes and convert to hours and minutes. (iii) Add if the place is east, subtract if west. (iv) Adjust past midnight or noon and, if needed, the date.

4. Standard time. For one time in a country, the local time of a central standard meridian is used, chosen in multiples of 7 degrees 30 minutes so the difference from GMT is a whole number of hours or half-hours. India uses 82 degrees 30 minutes E (through Mirzapur): 82.5 x 4 = 330 minutes, so IST = GMT + 5 h 30 min.

5. Time zones. Countries with a large east-west span, such as Russia, Canada and the USA, use more than one standard meridian. The world is divided into 24 major time zones.

6. International Date Line. Somewhere the date must change. The IDL runs approximately along 180 degrees, where time is 12 hours from Greenwich both ways, so the two sides differ by one day. Crossing it eastward, count one day back (Tuesday becomes Monday); crossing westward, one day forward (Tuesday becomes Wednesday).

Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line: key facts

Time and longitude: $360 d e g r ees = 24 h o u r s ∣15 d e g r ees = 1 h o u r ∣1 d e g r ee = 4 min u t es$; East ahead, west behind.
Time difference: Longitude difference x 4 minutes
Same side of 0: subtract
Opposite sides: add; Convert minutes to hours.
IST: $S t an d a r d m er i d ian 82 d e g r ees 30 min u t es E ∣82.5 x 4 = 330 min u t es ∣ I S T = GM T + 5 h 30 min$; Through Mirzapur.
Date line: Near 180 degrees
East crossing: one day back
West crossing: one day forward; Total difference of one day.

How to answer Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line questions

Time sums follow four steps.

1Find the longitude difference using the signs E and W.
2Multiply by 4 minutes.
3Add the result for a place east and subtract for a place west of the reference.
4Correct the clock and date if the time passes midnight.

Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Live match abroad

Suppose a match is played at a ground on 75 degrees W and Greenwich time is 3:00 pm.

What is the local time at the ground?

Try it first, then show the solutionSolution

75 x 4 = 300 minutes = 5 hours.
West is behind: 3:00 pm - 5 h = 10:00 am.

Answer: 10:00 am.

Memory hook: West minus.

Flight across the date line

Suppose a traveller flying east across the International Date Line reads the date as Tuesday just before crossing.

What day is it just after crossing?

Try it first, then show the solutionSolution

Eastward crossing counts one day back.
Tuesday becomes Monday.

Answer: Monday.

Memory hook: East goes back.

Tricks to remember Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line

Short memory aids. Say each one aloud once, then use it on a question the same day.

15 and 4
15 degrees an hour, 4 minutes a degree
Use it when: Core rule.
East ahead, West behind
Add east, subtract west
Use it when: Direction.
Date line back or forward
East back a day, west forward a day
Use it when: IDL.

Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Degrees per hour · 1 mark · MCQBoard level

The earth turns through how many degrees of longitude in one hour?
(A) 4
(B) 15
(C) 30
(D) 360

Try it first, then show the solutionSolution

360 / 24 = 15 degrees per hour (1 mark).

Answer: (B)

Example 2: Time at 60 degrees E · 2 marks · Very short answerBoard level

When Greenwich time is 12 noon, find the time at 60 degrees E. (2 marks)

Try it first, then show the solutionSolution

60 x 4 = 240 min = 4 hours (1 mark).
East is ahead: 4:00 pm (1 mark).

Answer: 60 x 4 = 240 minutes = 4 hours. The place is east, so the time is 4:00 pm.

Example 3: Opposite sides of Greenwich · 3 marks · Short answerHarder board level

When it is 10:00 am at 45 degrees E, find the time at 30 degrees W. (3 marks)

Try it first, then show the solutionSolution

Longitude difference = 45 + 30 = 75 degrees (opposite sides, add) (1 mark).
75 x 4 = 300 min = 5 hours (1 mark).
West is behind, so 10:00 am - 5 h = 5:00 am (1 mark).

Answer: The places lie on opposite sides of 0 degrees, so the difference is 45 + 30 = 75 degrees, i.e. 75 x 4 = 300 minutes = 5 hours. The second place is west, so it is behind: 10:00 am - 5 hours = 5:00 am on the same day.

Example 4: Assertion and reason: standard meridian · 1 mark · Assertion-reasonHarder board level

Assertion (A): A country's standard time differs from GMT by whole or half hours.
Reason (R): Its standard meridian is selected as a multiple of 7 degrees 30 minutes of longitude.
(A) Both true, R explains A
(B) Both true, R does not explain A
(C) A true, R false
(D) A false, R true

Try it first, then show the solutionSolution

A meridian that is a multiple of 7.5 degrees differs from GMT by a multiple of 30 minutes (1 mark).

Answer: (A)

Example 5: HOTS: across the date line · 5 marks · Long answerHOTS / competency

Place P lies at 170 degrees E and place Q at 170 degrees W, close to each other on either side of the International Date Line. When it is 9:00 am on Tuesday at P, find the local time and day at Q, showing the longitude difference, and explain the date. (5 marks)

Try it first, then show the solutionSolution

P and Q are on opposite sides of Greenwich, so add: 170 + 170 = 340 degrees (1 mark).
340 x 4 = 1,360 minutes = 22 hours 40 minutes (1 mark).
Q is west of P, so it is behind: 9:00 am Tuesday - 22 h 40 min (1 mark).
= 10:20 am on Monday (1 mark).
Check: Q is only 20 degrees (1 h 20 min) east of P across the line, so the clock is 1 h 20 min later, but the date is one day back (eastward crossing) (1 mark).

Answer: The places are on opposite sides of 0 degrees, so the longitude difference is 170 + 170 = 340 degrees, which is 340 x 4 = 1,360 minutes = 22 h 40 min. Q is west of P and therefore behind: 9:00 am Tuesday minus 22 h 40 min = 10:20 am Monday. This agrees with the date line rule: going east across the line from P to Q adds only 1 h 20 min on the clock (20 degrees x 4) but the date goes back one day.

Where students lose marks in Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line

The slips examiners see most often, and what to write instead.

Mistake: Adding for the west
Direction confusion.
Wrong working
New Orleans (90 degrees W): 12 + 6 = 6 pm.
Right working
West is behind: 12 - 6 = 6 am.
Do this: East add, west subtract.
Usually costs: 1 mark.
Mistake: Subtracting longitudes on opposite sides
Sign error.
Wrong working
45 degrees E and 30 degrees W: 45 - 30 = 15.
Right working
Add when on opposite sides: 45 + 30 = 75.
Do this: Check hemispheres first.
Usually costs: 1 mark.
Mistake: Reversing the date rule
Eastward and westward swapped.
Wrong working
Crossing east, a day is added.
Right working
Crossing east, one day back; west, one day forward.
Do this: East back.
Usually costs: 1 mark.

Exam tips

Write the longitude difference first, then the minutes, then the clock time.
State east or west and ahead or behind in words.
Always check the date after crossing midnight.

Practice questions on Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The International Date Line runs approximately along:
(A) 0 degrees
(B) 90 degrees E
(C) 180 degrees
(D) 82 degrees 30 minutes E
Hint
Opposite Greenwich.
Answer
(C)
Q2 · 2 marks · Very short answerBoard level
When it is 12 noon at Greenwich, find the time at 75 degrees W. (2 marks)
Hint
West is behind.
Answer
75 x 4 = 300 minutes = 5 hours behind: 7:00 am.
Q3 · 3 marks · Short answerHarder board level
Find the local time at 100 degrees E when it is 2:00 pm at 20 degrees E. (3 marks)
Hint
Same side.
Answer
Both are east of Greenwich, so the difference is 100 - 20 = 80 degrees. 80 x 4 = 320 min = 5 h 20 min. The place is further east, so it is ahead: 2:00 pm + 5 h 20 min = 7:20 pm.
Q4 · 3 marks · Short answerHarder board level
Explain why countries like the USA, Canada and Russia have several standard times. (3 marks)
Hint
Large east-west extent.
Answer
These countries stretch over a very wide range of longitude, so one standard meridian would make clock time differ much from sun time at the east and west ends. They therefore use more than one standard meridian, giving several time zones, as the USA does with seven.
Q5 · 4 marks · Short answerHOTS / competency
A traveller crosses the International Date Line westward on a Friday afternoon. State the day on arrival, explain the rule, and say why the date must change at some meridian. (4 marks)
Hint
West forward.
Answer
Crossing westward, one day is added: Friday becomes Saturday. The rule: eastward crossing counts one day back, westward one day forward. A date line is needed because time at 180 degrees differs by 12 hours from 0 degrees either way, so the two sides differ by a whole day and there must be a place where the date changes.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.How many minutes per degree?Show answer
4.
2.Which meridian is the IST meridian?Show answer
82 degrees 30 minutes E.
3.How many major time zones?Show answer
24.
4.Which city does the IST meridian pass?Show answer
Mirzapur.

Longitude and Time: Local Time, Standard Time, Time Zones and the International Date Line: frequently asked questions

What is local time?

Time at a place based on the position of the sun, set by its own meridian.

Why does time at 180 degrees differ by 12 hours?

Because 180 degrees x 4 minutes = 12 hours from 0 degrees.

Does the date line follow 180 degrees exactly?

It runs approximately along 180 degrees.