VBOXII units capture and log GPS time only - this is labelled as UTC time within the logged VBO file.
On the 31st of December 2016, a new leap second was added to GPS time, taking the current total offset between GPS and UTC time to 18 seconds. This means that the logged GPS time data will be 18 seconds behind UTC time.
There is no way of applying an offset to GPS time within this unit - the time is always fixed to GPS time, so there will never be any instances of time jumps within a captured data file.
What is the difference between GPS and UTC time?
UTC time is the time we all live by. It is affected by leap years and also leap seconds.
GPS time started on the 1st January, 1980, and was accurate to UTC at this point. Since then, there have been 18 leap seconds introduced, such that there is now an 18 second difference between GPS time and UTC time.
Wikipedia: ‘Because the Earth's rotation speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and unpredictable. Insertion of each UTC leap second is usually decided about six months in advance by the International Earth Rotation and Reference Systems Service. When needed to ensure that the difference between the UTC and UT1 (GMT) readings will never exceed 0.9 second.’
Instead of reprogramming each satellite’s internal clock with every leap second added, it was decided that an easier method would be to include the current number of leap seconds in the downloaded navigational data. GPS engines would then be able to apply this offset.