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Overview

Time Zones and Time Calculation

April 10, 2024
1 min read
time-zones IST GMT IDL

Why does Time Change?

As the Earth rotates from West to East, different longitudes face the Sun at different times.

The Math of Time

The Earth completes a 360∘360^\circ rotation in 24 hours.

  • 360∘÷24 hours=15∘ per hour360^\circ \div 24 \text{ hours} = 15^\circ \text{ per hour}.
  • Therefore, 1∘1^\circ change in longitude = 4 minutes.

Local Time vs. Standard Time

  • Local Time: Determined by the position of the Sun at a specific longitude.
  • Standard Time: A uniform time adopted by a country to avoid confusion.

Indian Standard Time (IST)

India uses the 82.5∘E82.5^\circ \text{E} longitude as its standard meridian.

  • IST is 5 hours 30 minutes ahead of Greenwich Mean Time (GMT).
Time difference=82.5∘×4 min=330 min=5.5 hours\text{Time difference} = 82.5^\circ \times 4 \text{ min} = 330 \text{ min} = 5.5 \text{ hours}

International Date Line (IDL)

The IDL is located approximately at 180∘180^\circ longitude.

  • Crossing it from East to West: Add a day (Sunday becomes Monday).
  • Crossing it from West to East: Subtract a day (Monday becomes Sunday).
Warning

The International Date Line is not a straight line. It zig-zags to avoid crossing through countries, so that a single country doesn’t have two different dates at the same time!