Tonight, at 23:59:59, the world's atomic clocks will run against the normal rules of time and tick for an extra second.
Why? Because the Earth is slowing down - and that's a bit of a fiddle for the internet.
Earlier this year it was announced that a "positive leap second" would be added to 30 June, seeing the clocks of institutions such as the Royal Observatory in Greenwich and the Observatoire of Paris read 23:59:60 before tocking into 1 July. This is to account for the gradual slowing of the Earth's rotation (blame the Moon) - a dilemma we could ignore, but given that our very perception of days, hours and minutes is defined by our planet's spin, scientists think it's enough of a deal to account for it with a leap second.
The last time the world's atomic clocks performed this momentary pause was on 30 June in 2012, and it caused a bunch of websites to crash as a result. Computer programs and code don't like having to account for an extra second in their day, with time references playing an important part in the back end technicalities of a great many procedures. Websites like Reddit, Gawker and LinkedIn all experienced issues due to the appearance of an extra second, as their systems became confused with time references being misaligned.
There are much bigger potential problems posed by the leap - with institutions that run on "to the second" information having to account for the slight adjustment, such as stock markets, satellite communications and telecoms.
Stock exchanges managed to plan ahead last time round, but there are fears that tonight might cause disruption: it's the first time that a leap second has been added mid-week, when the markets are live, since they started operating electronically. They've braced themselves, but you never know. Websites that encountered issues three years ago are similarly prepared. But if your email client starts kicking off tomorrow morning or your favourite internet forum is offline, it might have something to do with the leap second.
Best check on your emergency food stock, just in case.
(Images: Shutterstock; UFE)