Complex sentence: Since this system of correction was implemented in 1972,26 leap seconds have been inserted, the most recent on 30 June 2015 at 23:59:60 UTC.
Split into simpler sentences: Since this system of correction was implemented in 1972,26 such leap seconds have been inserted. The most recent one, the 26th, happened on 30 June 2015 at 23:59:60 UTC.