When a leap second is added, some computer software systems pay careful attention to the need to keep every tick of the clock unique. This is necessary so that software that logs events to millisecond accuracy don't post those events out of order.

For example as December 31, 2016 approached midnight the clock didn't pause for a second, so there weren't two instances of 23:59:59. Instead the last four (five) seconds evenly split the gain of a quarter second, resulting in offical time being logged as:

23:59:56.00

23:59:56.75

23:59:57.50

23:59:58.25

23:59:59.00

23:59:60.00 (aka Jan 1, 2017 00:00:00)