What times is it in the U.K.?

Global time zones (the red lines divide them) (Photo from CIA World Factbook)
Global time zones (the red lines divide them)

Time zones in Britain and dealing with the 24-hour clock

The entire world actually sets it clocks by British time, since Great Brtiain is the keeper of the global standard of UTC (which stands for Coordinated Universal Time—or, at least, it does when you write it in French).

However, the Brits usually refer to patrotically UTC as GMT, or Greenwich Mean Time, which is calculated at the Royal Observatory in Greenwich, on London's outskirts (and well worth a visit).

Practically speaking, however, the U.K. is five hours ahead of Eastern Standard Time.

That means when it's 1pm in New York and 10am in San Francisco, it's 6pm in London.

(Or, rather, it's 18:00 in London. » more on 24-hour time

What about the rest of Europe?

Ireland, Portugal, and Iceland, are all also on GMT.

Most of the rest of Western Europe is one hour ahead of the U.K.

Greece, the easterly parts of Eastern Europe, and Finland are are all two hours ahead of GMT.

The Daylight Savings Time wrinkle

Since every country has its own date when daylight savings time kicks in, there are often brief periods when these differences shift by an hour. 

Confusingly, GMT itself doesn't observe Daylight Savings Time, whereas the nations of the U.K. do, so during DST (from mid-spring to mid-autumn) all of Great Britain (save the Greenwich Royal Observatory) is on GMT +1.