[zokier]

> At the point they needed to be inserted, Unix systems carry on counting seconds as units of time as they pass

unix systems essentially will roll back clocks during leap seconds. here is handy dandy table as an example:

    Time                         tv_sec         tv_usec    
    2015-06-30T23:59:58.9Z       1435708798     900000
    2015-06-30T23:59:59.0Z       1435708799          0
    2015-06-30T23:59:59.1Z       1435708799     100000
    ...                          ...            ...          
    2015-06-30T23:59:59.9Z       1435708799     900000
    2015-06-30T23:59:60.0Z       1435708799          0
    2015-06-30T23:59:60.1Z       1435708799     100000
    ...                          ...            ...          
    2015-06-30T23:59:60.9Z       1435708799     900000
    2015-07-01T00:00:00.0Z       1435708800          0
    2015-07-01T00:00:00.1Z       1435708800     100000
    ...                          ...            ...          

I don't know about you but I wouldn't call that "counting seconds as units of time as they pass"