The small microchips known as “subscriber identity modules” or SIM cards that are required for mobile phones to log on to a phone network will soon be 25 years old. While mobile phones and network technology have progressed in leaps and bounds, SIM cards are still lodged in handsets.

Most people would feel they can count on one day comprising the same number of hours, minutes and seconds as the next. But this isn’t always the case – June 30 will be a second longer in 2015 with the addition of a leap second