Bill Johnson, age 12, of Birmingham, Ala., for his question:

WHEN WAS STANDARD TIME ESTABLISHED?

Standard time is a worldwide system of uniform time zones. Standard time was established after an international conference was held in 1884 in Washington, D.C.

A year earlier, in 1883, the railroads of the United States and Canada had adopted a system for standard time. Before the adoption, each city in the United States and Canada kept local time of its own meridian. It proved to be very confusing.

With the growth of railroads, great difficulties were discovered with local times. Too often, railroads that met in the same city ran on different times.

In worldwide standard time there are 24 zones. Twenty three zones are full zones and one is divided into two half zones. Each full zone is 15 degrees longitude wide.

The difference in time between that of any full zone and its neighbor is exactly one hour. Within each zone, all clocks keep the same time, except for some local variations.

From east to west, the standard time zones in the United States and Canada are Atlantic, Eastern, Central, Mountain, Pacific, Yukon, Alaska Hawaii and Bering.

There is a difference of four minutes for each degree of longitude, or a difference of an hour for every 15 degrees. Under standard time, the time kept in each zone is that,of the central meridian, or longitude line.

The central meridians are those 15 degrees, 30 degrees, 45 degrees, and so on, east or west of the prime meridian. The prime meridian, established at the international conference in 1884, passes through Greenwich, England.

In theory, the zone boundaries should extend seven and a half degrees on either side of the central meridian. In practice, the boundaries are irregular lines. This is to avoid inconvenient changes in time.  Today, nearly all of the world's nations keep standard time.

In 1918, Congress gave the Interstate Commerce Commission authority to establish limits for time zones in the United States. Congress transferred this authority to the Department of Transportation in 1967.

In summer, people in most states advance their clocks one hour to use daylight saving times. An act of Congress, which became effective in 1967, declared that daylight savings time must be used throughout a state or not at all. But an amendment to the act in 1972 allows states that lie in more than one time zone to use daylight time in one zone without using it in the other.

Canada does not have a federal standard time law. But its time zones generally have the same names as those used in the United States.