If an update of the leap seconds file is done any time, it will bring the file up to date. If your system has had a failure for longer than that, you will need to obtain a new file from our Web site. If you have had a failure since the announcement of a leap second, you will also need a new file. It is important, as with all project test data, to keep false data files made up for tests and simulations in a different filespace. That work was used to set up the earlier time standards.
Earth rotation is now slower than it was in that period, about a century ago.
Leap seconds are introduced into UTC to prevent midnight, Standard Time, from drifting too far from solar midnight. The result is for UTC to jump BACK a second in relation to UT1, which measures Earth rotation, so that the two are again close, and are closing towards each other. In the Toolkit, these happenings are expressed as the introduction of a leap second and a jump of about one second in the file of data for UT1-UTC, at the start of the next day. These operations are not visible to the user, but if the data files are out of date, the operations cannot take place reliably and error returns will issue.