Maarg Manthan · Topic 1.10

Local and Standard Time, International Date Line, Calendar

Geography › Earth's Origin, Interior and Motions · Topic 1.10

Local time is the time found from the position of the Sun at a place, so it changes by four minutes for every degree of longitude. To avoid confusion, countries adopt a standard time based on one chosen meridian, such as India's 82.5° E meridian, which is 5 hours 30 minutes ahead of Greenwich Mean Time. The International Date Line, running mostly along 180°, is where the calendar date changes by one day. Calendars, such as the Gregorian calendar and the Indian National Calendar, count days into months and years.

7 min read · UPSC Prelims · Geography · Earth's Origin, Interior and Motions

At a Glance

Local time Changes by 4 minutes per degree of longitude
Standard time Time of a chosen central meridian
IST 82.5° E; GMT plus 5 hours 30 minutes
Date line Follows about 180°; one day lost or gained
Calendar Gregorian for civil use; Indian National Calendar from 1957
Exam link UPSC Prelims, Geography: Earth’s Origin, Interior and Motions

Where Does It Fit?

This topic applies the Earth’s rotation and the grid of longitudes to the measurement of time. It follows the topics on the tilt of the axis and gives the practical rules for time zones, the date line and calendars that Prelims often tests through calculation. Previous topic: Inclination of the Earth’s Axis and Its Effects. Next topic: Solar and Lunar Eclipses.

What Are Local Time and Standard Time?

In short: every meridian has its own local time, so countries pick one meridian and follow its time.

The Earth rotates 360 degrees in 24 hours, so it turns 15 degrees in an hour or 1 degree in 4 minutes. Local time is fixed by the Sun: when the Sun is at its highest at a place, it is 12 noon local time. Places to the east see the Sun earlier and so have a later local time, while places to the west have an earlier time. A traveller going east must put the clock forward by 4 minutes for every degree, and going west must put it back.

Local time is not practical for a country, because every town would have a different time. A standard time is therefore chosen, usually along a meridian that is a multiple of 15 degrees or 7.5 degrees, near the middle of the country. Greenwich Mean Time (GMT), the local time on the Prime Meridian, was long the world reference, and has been replaced for scientific use by Coordinated Universal Time (UTC).

A sample calculation: when it is 12 noon at Greenwich, the time at 82.5° E is 82.5 divided by 15, which is 5.5 hours ahead, giving 5:30 in the evening. This is how Indian Standard Time is obtained.

What Are Standard Time and Time Zones?

Local time changes with every meridian, which would be confusing in daily life. Standard time is the time of a chosen longitude, adopted for a whole country or region. Most countries take their standard time from the central meridian of the country. Countries with a very large east-west span find a single time inconvenient and use several time zones: Russia has eleven, Canada six and the United States six main zones. A country with a large north-south extent but a small east-west extent, such as Chile, does not need multiple time zones.

Indian Standard Time is the time at 82.5 degrees E longitude, which passes close to Prayagraj in Uttar Pradesh and runs through Mirzapur. IST is therefore 5 hours 30 minutes ahead of GMT. India extends from about 68 degrees 7 minutes E to 97 degrees 25 minutes E, a span of nearly 30 degrees, so the Sun rises about two hours earlier in Arunachal Pradesh than in Gujarat, even though the whole country follows one time.

What Was Chaibagaan Time?

About 150 years ago, British planters introduced Chaibagaan time, or Bagaan time, in the tea gardens of Assam, which ran one hour ahead of IST so that daylight could be used better. After Independence Assam followed IST like the rest of India. The Assam administration has proposed returning to the earlier time to save energy and raise productivity, but the Government of India has not accepted such a proposal. The debate shows how a country with a wide east-west span can face pressure for more than one time.

What Is the International Date Line?

The International Date Line is an imaginary line that runs through the Pacific Ocean, roughly along the 180 degree meridian. The date changes by exactly one day when it is crossed. A traveller going from east to west across the line loses a day, and a traveller going from west to east gains a day. The reason is that 180 degrees E is GMT plus 12 hours and 180 degrees W is GMT minus 12 hours, so the two sides differ by 24 hours.

The line is not straight; it bends around island groups so that places of the same country or island group keep the same date. It cuts across Oceania, which includes Australia, Melanesia, Micronesia and Polynesia. Samoa, Kiritimati (Christmas Island) in Kiribati and Tonga are among the first places to welcome a new year, while Baker Island and Howland Island of the United States are among the last. As an analytical point, the 180 degree meridian was chosen because it passes mostly through open ocean, which keeps disruption to settled land to a minimum.

How Are Calendars Made?

A calendar counts days in a system based on the movements of the Earth and the Moon.

Calendar	Basis	Key facts
Julian	Solar year of 365.25 days	Introduced by Julius Caesar in 45 BCE; adds a leap day every four years, which makes the year slightly too long
Gregorian	Solar year of about 365.2425 days	Introduced by Pope Gregory XIII in 1582 to correct the Julian drift; now the civil calendar of most countries
Indian National Calendar (Saka)	Solar year, begins with the month Chaitra	Adopted from 22 March 1957 for official use along with the Gregorian calendar; Chaitra begins on 22 March, or 21 March in a leap year
Lunar and lunisolar	Lunar months of about 29.5 days	Islamic calendar is purely lunar; the Hindu calendars for festivals are lunisolar and add a leap month to keep pace with the seasons

The leap year rule of the Gregorian calendar is that a year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. So 2000 was a leap year, but 1900 was not. The rule keeps the calendar year close to the seasonal year, which is explained in Motions of the Earth: Rotation and Revolution and Their Effects.

Why Is This Topic Important?

Calculation Time questions are common
India One time zone for a wide country
Calendar Leap year and national calendar

What Came Next?

The next topic turns to the Sun, Earth and Moon again and explains the solar and lunar eclipses. Next topic: Solar and Lunar Eclipses.

Key People and Terms

Local time Time from the position of the Sun
Standard time Time of a chosen meridian
UTC Coordinated Universal Time
International Date Line Line where the date changes
Gregorian calendar Calendar introduced in 1582
Saka era Basis of the Indian National Calendar

Exam Corner

Points to Remember

Time changes by 4 minutes for each degree.
IST is GMT plus 5 hours 30 minutes.
Crossing the date line from west to east: subtract a day.
Gregorian calendar: 1582.
Indian National Calendar: from 22 March 1957.

Do Not Confuse With

Local and standard time Time of each place versus time of one chosen meridian
GMT and UTC Old solar reference versus the atomic-clock standard
Julian and Gregorian calendars Older and less accurate versus the one now in use

Memory Hook

Fifteen degrees, one hour; east is more, west is less.

Mains Angle

Questions ask for the case for and against more time zones in India. These points add depth.

India Two-hour solar gap between east and west
Case for Better use of daylight, work and school hours
Case against Coordination, railways and administration

A Question You May Face

An original practice question, not a past paper question.

“Discuss the case for and against more than one time zone in India.”

How to Answer

Basis Longitude and time.
Case for Northeast daylight, energy.
Case against Coordination and safety.
Verdict Balance of both.

GS Relevance

Prelims: time calculation, IST, date line, calendars. GS Paper 1: motions of the Earth and time zones.

Frequently Asked Questions

What is the difference between local and standard time?

Local time is based on the position of the Sun at a place and changes by four minutes for every degree of longitude. Standard time is the local time of a chosen meridian, which is adopted over a whole country or region to avoid confusion caused by different local times.

What is Indian Standard Time?

Indian Standard Time is the local time at 82.5 degrees east longitude, which passes near Mirzapur in Uttar Pradesh. It is 5 hours 30 minutes ahead of Greenwich Mean Time or Coordinated Universal Time and is used throughout India, although the country spans nearly 30 degrees of longitude.

What is the International Date Line?

The International Date Line is an imaginary line that runs mostly along the 180 degree meridian from pole to pole, with bends to avoid land. Crossing it from west to east, a traveller subtracts one day, and crossing it from east to west, adds one day to the calendar.

How is a leap year determined?

In the Gregorian calendar, a year is a leap year if it is divisible by 4, except century years, which must be divisible by 400. Thus 2000 was a leap year, but 1900 was not. The leap day, 29 February, keeps the calendar in step with the seasons.

What is the Indian National Calendar?

The Indian National Calendar, based on the Saka era, was adopted for official use from 22 March 1957 alongside the Gregorian calendar. Its year begins with the month Chaitra on 22 March, or on 21 March in a leap year, and has 365 days in a normal year.

PYQ Practice — Statement Analysis

1 Indian Standard Time is based on the 82.5 degrees east meridian.

True

It is GMT plus 5 hours 30 minutes.

2 A traveller crossing the date line from west to east gains a day.

False

The traveller loses, or subtracts, a day.

3 The year 1900 was a leap year in the Gregorian calendar.