You've probably heard these short beeps known as 'pips' - the Royal Observatory in Greenwich England has been broadcasting these hourly time signals since 1924. You can hear them before the BBC's hourly newscasts.
Click here for a 360 degree panorama of the Greenwich Meridian
The pips mark "Greenwich Mean Time" or GMT. It's come to be known as Universal Time. Times in cities around the world are often expressed as a number of hours "ahead of GMT" or "behind GMT".
So here's the question: When it's noon in the city of Los Angeles, what time is it in Mecca, Saudi Arabia? The answer may depend on whom you ask, some Muslim scientists are calling for abandoning the GMT standard.
In any case, you've got about 5 minutes to come up with the answer.