Coordinated Universal Time (UTC) follows TAI (see below) exactly except for an integral number of seconds, presently 32. These leap seconds are inserted on the advice of the International Earth Rotation Service (IERS) to ensure that, on average over the years, the Sun is overhead within 0.9 seconds of 12:00:00 UTC on the meridian of Greenwich. UTC is thus the modern successor of Greenwich Mean Time, GMT, which was used when the unit of time was the mean solar day.

International Atomic Time (TAI) is calculated by the BIPM from the readings of more than 200 atomic clocks located in metrology institutes and observatories in more than 30 countries around the world. TAI is made available every month in the BIPM Circular T. We estimate that TAI does not lose or gain with respect to an imaginary perfect clock by more than about one tenth of a microsecond (0.000 000 1 second) per year.

Warning: Please note that the time displayed here is for demonstration only and may be affected by transmission delay, for which an estimate is given above, and by any workload of your computer. The image on your screen may not therefore represent the actual values of UTC or TAI. It is also possible that the time displayed can be affected by your own Java Virtual Machine.