Local time and time zones — Geography

In this lesson you will learn

Calculate the local time difference from a difference in longitude, and longitude from a time difference
Find the time zone of a place in Roman numerals with the 7.5° remainder rule
Compare local, zone and official time and their differences
Work out the time and the date across the date line

One winter evening at 8 p.m., Murad calls his cousin in New York from Baku. His cousin has not even had lunch — it is 11 a.m. there! Why do clocks show different times in different places at the very same moment? The answer lies in the Earth's rotation.

Local time

The Earth turns 360° in 24 hours. So it turns 15° every hour and 1° every 4 minutes. The Sun stands highest over each meridian at noon, and at that moment local noon begins on that meridian. Because the Earth spins from west to east, places farther east are always ahead in time.

Definition

Local time

The time set by the position of the Sun relative to a given meridian. All places on one meridian share the same local time; it changes by 4 minutes for every 1° you move east or west.

Δt = Δλ × 4 min

where:

Δtdifference in local time
Δλdifference in longitude between two places, in degrees

1° of longitude = 4 minutes, 15° = 1 hour

Worked example

Baku lies at about 50° E. When it is local noon in Greenwich (0°), what is the local time in Baku?

Show solution

Difference in longitude: 50° − 0° = 50°.
Time difference: 50 × 4 min = 200 min = 3 h 20 min.
Baku is to the east, so we add: 12:00 + 3:20 = 15:20.

There is a simple rule for the difference in longitude (Δλ): if both places are in the same hemisphere (both at east or both at west longitude), subtract the longitudes; if one is east and the other west, add them. For example, between 25° E and 25° W, Δλ = 25° + 25° = 50°, so the local time difference is 50 × 4 = 200 min, i.e. 3 h 20 min. Azerbaijani exam papers (DİM) abbreviate longitudes: ş.u. means east longitude and q.u. means west longitude.

Example: finding longitude from a time difference

Baku lies at about 50° E.
1) The local time of town A is 2 h 48 min behind the local time of Baku.
2) The local time of place B is 4 h 20 min behind that of Baku.
Work out the longitude of A and of B.

Show solution

1) 2 h 48 min = 168 min; 168 ÷ 4 = 42° of longitude.
The time is behind → A lies west of Baku: 50° − 42° = 8° E.
2) 4 h 20 min = 260 min; 260 ÷ 4 = 65°.
50° − 65° = −15°. The result is negative, so B is west of the Greenwich meridian: 15° W.
Check: 50° + 15° = 65° (different hemispheres, so add), 65 × 4 = 260 min ✓

Time zones: from 0 to XXIII

If every town lived by its own local time, train and flight timetables would be chaos. So the Earth’s surface is divided along the meridians into 24 time zones, each 15° wide (24 × 15° = 360°). Counting starts at the Greenwich meridian: the strip on both sides of it, from 7.5° W to 7.5° E, is zone 0. The other zones are numbered eastwards in Roman numerals: I, II, III … XXIII. Zone 0 is sometimes also written as XXIV. This is exactly how Azerbaijani textbooks and the DİM entrance tests show the zones.

Definition

Zone (standard) time

The local time of the central meridian that runs through the middle of a zone; the whole zone uses it. The central meridian of zone n is 15° · n (0°, 15°, 30°, 45° …). Neighbouring zones differ by exactly 1 hour.

Definition

Official (legal) time

The time a country adopts by law. It may differ from zone time: some countries set their clocks one hour ahead of zone time all year, others move them forward in summer (daylight saving time), and parts of large countries live by a neighbouring zone’s time.

n = λ ÷ 15° (if the remainder > 7.5°, take n + 1); for W longitude N = 24 − n

where:

λthe longitude of the place, in degrees
nthe whole part of the division (corrected for the remainder); the zone number at east longitude
Nthe zone number at west longitude; 24 means zone 0

The DİM method: zone borders lie 7.5° from the central meridian, so a remainder larger than 7.5° means the place is already in the next zone.

1
Divide
Divide the longitude by 15°: write down the whole part (n) and the remainder (r).
2
Check the remainder
If r > 7.5°, add 1 to n; if r < 7.5°, n stays the same.
3
Mind the hemisphere
At east longitude the answer is n (0–XII). At west longitude it is 24 − n (XII–XXIII; 24 means 0).
4
Write it in Roman numerals
For example, 7 → VII, 22 → XXII. Time relative to Greenwich: n hours ahead in the east, 24 − N hours behind in the west.

Zone	Central meridian	Borders	Relative to Greenwich
0 (XXIV)	0°	7.5° W – 7.5° E	0 h
I	15° E	7.5° – 22.5° E	+1 h
II	30° E	22.5° – 37.5° E	+2 h
III	45° E	37.5° – 52.5° E	+3 h
IV	60° E	52.5° – 67.5° E	+4 h
XII	180°	172.5° E – 172.5° W	±12 h
XIII	165° W	172.5° – 157.5° W	−11 h
XXII	30° W	37.5° – 22.5° W	−2 h
XXIII	15° W	22.5° – 7.5° W	−1 h

Zones are counted eastwards from Greenwich, so the zones of the Western Hemisphere have large numbers (XIII–XXIII). Zone XII is cut in two by the 180° meridian: both halves show the same clock time but different dates.

Example: the time zone of a place

Find the time zones of these places: 1) 52° E; 2) 98° E; 3) 41° W; 4) 129° W.

Show solution

1) 52 ÷ 15 = 3, remainder 7° < 7.5° → zone III.
2) 98 ÷ 15 = 6, remainder 8° > 7.5° → 6 + 1 = 7 → zone VII.
3) 41 ÷ 15 = 2, remainder 11° > 7.5° → 3; west longitude: 24 − 3 = 21 → zone XXI (3 h behind Greenwich).
4) 129 ÷ 15 = 8, remainder 9° > 7.5° → 9; 24 − 9 = 15 → zone XV (9 h behind).

DİM-style task (5 options)

On a diagram, place A is at 50° W, place B at 25° E and place C at 110° E. Find the time zones of A, B and C and the zone-time difference between A and C.
A) XXI, II, VII – 10 h
B) XXII, I, VII – 9 h
C) III, II, VIII – 5 h
D) XXI, I, VIII – 11 h
E) XX, II, VII – 11 h

Show solution

A: 50 ÷ 15 = 3, remainder 5° < 7.5° → 3; west longitude: 24 − 3 = XXI (3 h behind).
B: 25 ÷ 15 = 1, remainder 10° > 7.5° → 1 + 1 = II.
C: 110 ÷ 15 = 7, remainder 5° → VII (7 h ahead).
A and C are in different hemispheres: 3 + 7 = 10 h.
Correct answer: A. (Options B and D ignore the remainder of place B: 10° > 7.5°.)

City (longitude)	Zone (λ ÷ 15°)	Official time	When it is 12:00 in Baku
London (0°)	0	UTC+0 (winter)	08:00
New York (74° W)	XIX	UTC−5 (winter)	03:00
Istanbul (29° E)	II	UTC+3	11:00
Moscow (38° E)	III	UTC+3	11:00
Baku (50° E)	III	UTC+4	12:00
Beijing (116° E)	VIII	UTC+8	16:00
Tokyo (140° E)	IX	UTC+9	17:00

Official time is not always equal to the zone number: in Istanbul and Baku it is 1 hour ahead of zone time. London and New York move their clocks one hour forward in summer; Azerbaijan and Türkiye have not done so since 2016.

Travel problem

In winter a plane leaves Baku (UTC+4) for London (UTC+0) at 10:00. The flight takes about 6 hours. What time will it land, London time?

Show solution

When it is 10:00 in Baku, it is 10:00 − 4 h = 06:00 in London.
The flight takes 6 hours: 06:00 + 6 h = 12:00 London time.
To the passenger it feels as if the trip took only 2 hours!

Interactive

Loading simulation…

Move along the meridians and watch how local time changes with longitude.

Local time difference and zone time difference

The local time difference of two places is proportional to their difference in longitude and is exact to the minute (Δλ × 4 min). The zone time difference is always a whole number of hours and is found from the zone numbers: if both places are in the same hemisphere, subtract the numbers; if one is east (n) and the other west (N), the difference is n + (24 − N) hours. So the local time difference can be greater than, smaller than or equal to the zone time difference, and DİM tests this often.

Example: local time and zone time

1) At a place at 55° E the zone time is 12:00. What is the local time there?
2) In Baku (50° E) the official clock (UTC+4) shows 12:00. What is Baku’s local time?

Show solution

1) 55 ÷ 15 = 3, remainder 10° > 7.5° → zone IV; its central meridian is 4 × 15° = 60° E.
The place is 60° − 55° = 5° west of the central meridian → local time is 5 × 4 = 20 min behind: 11:40.
2) UTC+4 is the time of the 60° meridian. Baku is 10° west of it: 10 × 4 = 40 min → local time 11:20.
So local noon in Baku comes at about 12:40 official time.

DİM-style task: comparing local and zone time differences

Choose the pairs of longitudes whose local time difference is greater than their zone time difference.
1) 10° E and 10° W
2) 7° W and 22° E
3) 30° E and 30° W
4) 50° E and 97° E
5) 5° E and 40° E
6) 20° E and 52° E
A) 1, 3, 5 B) 2, 4, 6 C) 1, 2, 4 D) 3, 4, 6 E) 2, 5, 6

Show solution

1) Δλ = 20° → 1 h 20 min; zones I and XXIII → 1 + 1 = 2 h (local is smaller).
2) Δλ = 29° → 1 h 56 min; zones 0 and I → 1 h (local is greater).
3) Δλ = 60° → 4 h; zones II and XXII → 4 h (equal).
4) Δλ = 47° → 3 h 8 min; zones III and VI → 3 h (local is greater).
5) Δλ = 35° → 2 h 20 min; zones 0 and III (40 ÷ 15 = 2, remainder 10°) → 3 h (local is smaller).
6) Δλ = 32° → 2 h 8 min; zones I and III → 2 h (local is greater).
Correct answer: B) 2, 4, 6.

The International Date Line

Somewhere on the planet a new day has to begin. That place is the International Date Line, which runs mostly along the 180° meridian through the Pacific Ocean. It bends in places so as not to split islands and countries. When you cross it going west (from America to Asia), you add one day to the calendar; going east, you subtract one day.

In diagram tasks a longitude may pass 180°. If, counting eastwards, you get more than 180°, subtract it from 360°, and the place is at west longitude: for example, 150° E + 60° = 210° → 360° − 210° = 150° W. Remember: west of the 180° meridian (the Asian side) the date is one day ahead of the date east of it (the American side).

Example: across the date line

1) On 21 March the local time at place K, at 150° E, is 10:00. What are the local time and the date at place M, 60° east of K?
2) On 5 May the local time at place L, at 170° W, is 22:00. What are the local time and the date at place N, at 160° E?

Show solution

1) Longitude of M: 150° + 60° = 210° → 360° − 210° = 150° W.
M is to the east: time is 60 × 4 = 240 min = 4 h ahead → 10:00 + 4 = 14:00.
M is on the American side of the 180° meridian → the date is one day back: 20 March, 14:00.
2) From L to 180° is 10°, and from there to 160° E another 20° → N is 30° west of L: time is 2 h behind → 20:00.
We crossed 180° going west → the date moves one day forward: 6 May, 20:00.

DİM-style task: the false statement about a diagram

On 21 June the local time at place K, at 120° E, is 18:00. At the same moment the local time at place L on the same parallel is 06:00, and at place M it is 21:00. Find the false statement.
A) L is at 60° W.
B) The difference in longitude between K and M is 45°.
C) M is in the Eastern Hemisphere.
D) K is in time zone XI.
E) It is also 21 June at L.

Show solution

L: 18 − 6 = 12 h behind → 12 × 15° = 180° to the west → 120° − 180° = −60° → 60° W (A is true).
M: 21 − 18 = 3 h ahead → 45° to the east → 120° + 45° = 165° E (B and C are true).
K: 120 ÷ 15 = 8 → zone VIII, not XI (D is false).
It is 06:00 at L and 18:00 at K on the same day; L lies west of Greenwich without crossing the 180° line (E is true).
Correct answer: D.

Key points

The Earth turns 15° every hour and 1° every 4 minutes; places to the east are ahead in time.
Time zones are numbered eastwards from Greenwich: 0 (XXIV), I, II … XXIII; each zone is 15° wide.
Zone: λ ÷ 15°, add 1 if the remainder > 7.5°; at west longitude subtract from 24.
The local time difference is exact to the minute (Δλ × 4 min); the zone time difference is in whole hours.
Azerbaijan is geographically in zone III, but its official time is UTC+4.
Crossing the date line westwards adds a day; crossing eastwards takes a day away.

Check yourself

12 questions. Every correct answer earns XP.

1 / 12

How many degrees does the Earth turn in one hour?