If it's 9:05, it must be Baltimore; but if it's 8:54:58, it's N.Y.C.

This month marks the hundredth birthday of something we take for granted, but which runs our lives. Did you guess standard time? You win a digital alarm clock.

On Nov. 18, 1883 (popularly known as ''the day with two noons''), most North American railroads voluntarily switched their schedules to Standard Railway Time. The new system sliced the continent into five ''zones'' - Intercolonial, Eastern, Central, Mountain, and Pacific - each of which had a uniform time within its boundaries.

Most cities soon followed suit. This system, though much modified, is what today marks when we get up, when we eat meals, and when the 6 o'clock news comes on.

''Time zones,'' points out Carlene Stephens, curator of a Smithsonian exhibit commemorating the switch, ''are a man-made invention just imposed on society.''

They were imposed upon, however, to bring order out of chaos. Until 1883, time was so unruly that setting your watch was an adventure.

All across the United States, local time was determined by the position of the sun relative to each location - which meant it was very local indeed. When it was 12:00 in Washington, D.C., it was 12:24 in Boston. New York City was 10 minutes and 2 seconds ahead of Baltimore. Connecticut alone had five time zones.

As long as the nation's pace was relatively slow, with the economy largely agricultural, such a crazy-quilt system worked fine, Ms. Stephens says. But as rails and telegraph wires snaked across the nation, binding communities more closely together, the multitude of times became a nuisance.

Scientists began pressing for time reform in the early 1800s. But it took the railroads - the most powerful economic and political force in 19th-century America, Ms. Stephens says - to do the job.

The General Time Convention, a railway trade association that coordinated timetables, began drawing up a new time system in 1881. In the spring of 1883, railway owners voted to adopt this plan, dubbed ''Railway Standard Time.''

William F. Allen, head of the Time Convention, traveled the country lobbying local politicians, journalists, and other civic leaders on the merits of the new plan. He was largely successful. One year after the railroads themselves switched, 85 percent of towns with more than 10,000 people had also adopted standard time.

''No, the public didn't really care that much,'' says curator Stephens. ''It was a period not as preoccupied with punctuality as we are now.''

Some did object. A few communities resented the all-powerful railroads imposing their standard of time on everyone. Others just felt their local autonomy was being threatened. A few religious leaders felt standard time interfered with the laws of God and natural order.

Standard time is ''chimerical, utterly impractical,'' said Adm. John Rodgers, in an embarrassingly inaccurate prediction enshrined in the Smithsonian exhibit.