National Physical Laboratory


Answers to a range of frequently-asked questions about clocks and timekeeping can be found here.

General Time FAQs

Clock FAQs

Leap Second & Leap Year FAQs

  • Yes, 2000 was a leap year. Since 1752, in this country, years exactly divisible by 100 are only leap years when they are also exactly divisible by 400.
  • The International Earth Rotation and Reference Systems Service (IERS), based at the Paris Observatory, announces twice yearly whether or not there will be a leap second at the end of the following June or December.
  • As UTC is running about one second per year faster than time based on the Earth's rotation (Greenwich Mean Time, or Universal Time), an extra second is occasionally inserted in the UTC time scale to let it fall behind and start catching up again.


Miscellaneous FAQs

  • The time displayed on a well designed sundial will vary from the mean time during the year, up to 16 minutes ahead (November 3), and up to 14 minutes behind (February 11), only agreeing around April 15, June 13, September 1 and December 25th. Subject to these corrections a good sundial can be read to within a few minutes accuracy.
  • To find the time in Hong Kong relative to the UK you need to know the time zone of Hong Kong and whether either country is on summer time (also known as daylight-saving time).
  • GPS has its own date and time scale for expressing satellite positions, based on counting weeks, and seconds within a week. To limit the size of the numbers used in the data and calculations the GPS Week Number is a ten-bit count in the range 0-1023, repeating every 1024 weeks.
  • The gigasecond is the only scientifically-preferred time interval that is passed in adult life.


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