Time zones are measured from the Prime Meridian (the 0° line of latitude). The time at this 0° line is called Universal Time (UT) or Greenwich Mean Time (GMT). Think of it as being the official time of Earth. So, if other planets asked Earth what time it was, Earth would give them the time along the Prime Meridian. With the Greenwich Meridian in England as the starting point, each 15° to the east and west mark a new time zone.
Before time zones, cities set their clocks to the local position of the sun. This worked until steam engines, telegraphs, and railroads made it possible to travel fast enough over long distances to require the constant re-setting of timepieces.
Ohio is in the Eastern Standard Time (EST) zone. EST is five hours behind GMT and is written as GMT –5. We've all heard the saying "Spring Ahead and Fall Behind", and we all dread it when we have to lose an hour of sleep the night we move our clocks forward.
Daylight-Saving Time (DST) is observed in Ohio when time is shifted forward one hour at 2:00am on the second Sunday in March. This puts Ohio just four hours behind Greenwich (GMT –4). Time goes back an hour at 2:00am on the first Sunday in November. This returns Ohio to GMT –5. For example, in the fall, when it is 2:30pm in England, it is 9:30am in Westerville, and 11:30pm in Japan.
When it is noon along the Prime Meridian, it is midnight along the International Date Line. Regions east of the IDL are one calendar day earlier than regions to the west.
The GMT never shifts since it tracks the time that it takes Earth to rotate from noon to noon. Much of South America, Africa, Asia, and Australia do not observe DST.
The concept of Daylight-Savings Time was first advocated in 1907 by William Willett, a British builder. Willett "invented" DST during a pre-breakfast horseback ride where he was dismayed by how many Londoners were still asleep. Also an avid golfer, he disliked having to cut short his round at dusk. DST has been used in the U.S. and Europe since World War I (1914-1918), when it was adopted to save fuel.
William Willett doesn't want you to sleep.
Until 2007, DST started the first Sunday of April and ended the last Sunday of October in most of the U.S. except for Hawaii and Arizona, who don't observe DST. The change to March/November was part of the Energy Policy Act of 2005. So, if it seems like it "used" to be dark when you went Trick-or-Treating... don't worry, you aren't becoming one of those people that pine away for the past... it honestly DID used to be dark out for Halloween because DST ended late October.