The time difference between UTC and GPS is adjusted every 1 – 6 years. The last adjustment took place on December 31st 2016.
GPS is now ahead of UTC by 18 seconds.
Further information regarding Trimble receivers and POSpac processing can be found here:
To adjust your offset in snapSHOT, use the menu system: 4-Config -> 6 SnapSHOT configuration -> 1-Times configuration
Then adjust “Difference between UTC and GPS times” value to +18 Seconds and press Apply button.