Risk of an unprecedented negative adjustment prompts plan to let UTC drift by as much as an hour
The prospect of a negative leap second – combined with decades of irritation over leap seconds generally – looks set to inspire a bit of global procrastination among international timekeepers. Next month, the General Conference on Weights and Measures is set to vote on effectively calling time on leap seconds. A draft resolution [PDF] would make Coordinated Universal Time (UTC) continuous from May 20, 2027, and allow the difference between UTC and UT1, a measure of time based on Earth's rotation, to reach as much as one hour. Leap seconds keep UTC, which is based on atomic clocks, within 0.9 seconds of time determined by the slightly wobbly rotation of our home planet. Since the system was introduced in 1972, 27 leap seconds have been added to UTC. The urgency comes partly from a slight increase in Earth's rotational speed in recent years, raising the possibility of an adjustment never attempted before: a negative leap second. Experts convened by international timekeeping bodies estimated a 30 percent probability that one would be needed by 2035. Many computer systems were designed on the assumption that a leap second would always be added rather than removed, explained Setnam Shemar, principal scientist at the UK's National Physical Laboratory (NPL). "Most digital architectures and network time servers are hardcoded under the assumption that leap seconds are strictly additive. Losing a second could trigger severe disruption across critical national infrastructure, including telecommunications, electricity grids, and satellite navigation systems," he said. The risks posed by a negative leap second – and leap seconds generally – are behind the proposal to let UTC diverge from rotational time by as much as an hour, which is due to be put to a vote in October. Shemar said the proposal would remove the need for further leap-second adjustments for many years. "This would remove the risks entirely for a very long time. Next month's resolution is likely to pass since it will be based on the outcome of a majority vote of the national delegates from many different countries, and most of them have been keen to end leap seconds for several years due to the risks posed to national infrastructure." UTC would be unlikely to diverge from rotational time by a full hour for at least a thousand years, Shemar said. If humankind lasts that long, it might be the least of our worries. ®
