Standard time and the Date Line

Standard time. If every town kept its own local time, clocks in one country would all differ. So a country takes the local time of a central meridian, its standard meridian, as the standard time for the whole country.

• The standard meridian is chosen as a multiple of 7°30', so that the difference from Greenwich Mean Time (GMT) is a whole number of hours or half-hours (7°30' × 4 minutes = 30 minutes).
• Indian Standard Time (IST) is the local time of 82°30' E, which passes through Mirzapur. IST is GMT + 5 hours 30 minutes.
• Countries with a very large east-west spread use more than one standard meridian and time zone, such as Russia, Canada and the United States.
• The world has 24 major time zones.

The International Date Line:

• Somewhere the date must change. The International Date Line runs roughly along the 180° meridian, which is 12 hours from Greenwich whichever way you travel.
• A traveller crossing it going east turns the calendar back a day (Tuesday becomes Monday).
• A traveller crossing it going west moves forward a day (Tuesday becomes Wednesday).

Example (new): a country whose standard meridian is 120° E is 120 × 4 = 480 minutes = 8 hours ahead of GMT. When it is 10 a.m. at Greenwich, it is 6 p.m. there, and 3:30 p.m. in India.