econds are extra seconds added or deducted from TAI to keep UTC within 0.9 second of the earth's orbit-a process managed by the International Earth Rotation and Reference Systems (IERS) service, which is responsible for monitoring the earth's rotation. IERS announces, about 5 months ahead of time (in a document called Bulletin C), the dates of any upcoming leap second. Usually, IERS announces that it will insert an extra second (as the earth's rotation is generally slowing down) on 30 June or 31 December. As of 1 January 2017 UTC, there have been 37 leap seconds, which means that UTC time is 37 seconds behind TAI. In the normal course of events, there is a single leap second added every few years. Interestingly, adding or subtracting these extra seconds means that the last minute on the day of the leap event could have either 58, 59, 61, or 62 seconds. So, assuming the addition of a single leap second, at the forecast time of the insertion, a clock displaying UTC can read 23:59:60 for one ad- ded leap second (or 23:59:61 for two). But another clock displaying TAI would still only have 60 seconds in that minute, meaning that for one added leap second, UTC would fall one more second behind TAI. It is this adjusted UTC time that is the basis for the dates and times used in our daily life and is a replacement for the more commonly quoted GMT. Technically, there is only one UTC across the world, and the time from every atomic clock is only a local approximation of UTC, known as UTC(k) (where k is a name of the laboratory that manages the clock). The interesting fact about UTC is that only an approximate value for it is known in real time. The single "official" UTC is determined by post-processing masses of data collected from atomic clocks a couple of weeks after the end of the month. Once a month, Circular T from the BIPM gives us a readout of how closely aligned all the atomic clocks participating in the scheme are to "real" UTC. To quote from the BIPM website: BIPM Circular T is the monthly publication of the Time 