Dennis D. McCarthy is a former Director of Time at the United States Naval Observatory, the leading authority in the US for astronomical and timing data. He has led and been a member of various Commissions and Working Groups within the International Astronomical Union and has authored and edited numerous publications dealing with fundamental astronomy, time, and Earth orientation. P. Kenneth Seidelmann is a research professor of astronomy at the University of Virginia and is a former Director of Astrometry at the US Naval Observatory. He has led and been a member of a Division, various Commissions, and Working Groups of the International Astronomical Union, has co-authored two other books: Fundamentals of Astrometry (Cambridge, 2004) and Celestial Mechanics and Astrodynamics (2016), and is co-editor of the Explanatory Supplement to the Astronomical Almanac (2012).

'Why do we add 1 second to our clocks at midnight at the end of some years, or at the end of June in others? Why don't we subtract 1 second sometimes instead? … You will find the answers to these and many more questions in this excellent book, written by two experts who worked on the practical aspects of these topics at the US Naval Observatory … The second edition brings these subjects right up to date, and investigates the possible future developments in timekeeping.' L. V. Morrison, The Observatory

• Preface; 1. Time: pre-twentieth century; 2. Solar time; 3. Ephemerides; 4. Variable Earth rotation; 5. Earth orientation; 6. Ephemeris time; 7. Relativity and time; 8. Time and cosmology; 9. Dynamical and coordinate time scales; 10. Clock developments; 11. Microwave atomic clocks; 12. Optical atomic standards; 13. Definition and role of a second; 14. International Atomic Time (TAI); 15. Coordinated Universal Time (UTC); 16. Time in the solar system; 17. Time and frequency transfer; 18. Modern Earth orientation; 19. International activities; 20. Time applications; 21. Future of time keeping; Acronyms; Glossary.

Dennis D. McCarthy is a former Director of Time at the United States Naval Observatory, the leading authority in the US for astronomical and timing data. He has led and been a member of various Commissions and Working Groups within the International Astronomical Union and has authored and edited numerous publications dealing with fundamental astronomy, time, and Earth orientation. P. Kenneth Seidelmann is a research professor of astronomy at the University of Virginia and is a former Director of Astrometry at the US Naval Observatory. He has led and been a member of a Division, various Commissions, and Working Groups of the International Astronomical Union, has co-authored two other books: Fundamentals of Astrometry (Cambridge, 2004) and Celestial Mechanics and Astrodynamics (2016), and is co-editor of the Explanatory Supplement to the Astronomical Almanac (2012).

'Why do we add 1 second to our clocks at midnight at the end of some years, or at the end of June in others? Why don't we subtract 1 second sometimes instead? … You will find the answers to these and many more questions in this excellent book, written by two experts who worked on the practical aspects of these topics at the US Naval Observatory … The second edition brings these subjects right up to date, and investigates the possible future developments in timekeeping.' L. V. Morrison, The Observatory

• Preface; 1. Time: pre-twentieth century; 2. Solar time; 3. Ephemerides; 4. Variable Earth rotation; 5. Earth orientation; 6. Ephemeris time; 7. Relativity and time; 8. Time and cosmology; 9. Dynamical and coordinate time scales; 10. Clock developments; 11. Microwave atomic clocks; 12. Optical atomic standards; 13. Definition and role of a second; 14. International Atomic Time (TAI); 15. Coordinated Universal Time (UTC); 16. Time in the solar system; 17. Time and frequency transfer; 18. Modern Earth orientation; 19. International activities; 20. Time applications; 21. Future of time keeping; Acronyms; Glossary.

Dennis D. McCarthy is a former Director of Time at the United States Naval Observatory, the leading authority in the US for astronomical and timing data. He has led and been a member of various Commissions and Working Groups within the International Astronomical Union and has authored and edited numerous publications dealing with fundamental astronomy, time, and Earth orientation. P. Kenneth Seidelmann is a research professor of astronomy at the University of Virginia and is a former Director of Astrometry at the US Naval Observatory. He has led and been a member of a Division, various Commissions, and Working Groups of the International Astronomical Union, has co-authored two other books: Fundamentals of Astrometry (Cambridge, 2004) and Celestial Mechanics and Astrodynamics (2016), and is co-editor of the Explanatory Supplement to the Astronomical Almanac (2012).

'Why do we add 1 second to our clocks at midnight at the end of some years, or at the end of June in others? Why don't we subtract 1 second sometimes instead? … You will find the answers to these and many more questions in this excellent book, written by two experts who worked on the practical aspects of these topics at the US Naval Observatory … The second edition brings these subjects right up to date, and investigates the possible future developments in timekeeping.' L. V. Morrison, The Observatory

• Preface; 1. Time: pre-twentieth century; 2. Solar time; 3. Ephemerides; 4. Variable Earth rotation; 5. Earth orientation; 6. Ephemeris time; 7. Relativity and time; 8. Time and cosmology; 9. Dynamical and coordinate time scales; 10. Clock developments; 11. Microwave atomic clocks; 12. Optical atomic standards; 13. Definition and role of a second; 14. International Atomic Time (TAI); 15. Coordinated Universal Time (UTC); 16. Time in the solar system; 17. Time and frequency transfer; 18. Modern Earth orientation; 19. International activities; 20. Time applications; 21. Future of time keeping; Acronyms; Glossary.