Paul, ZL4TT recently got in touch about syncing FT8 clock without internet connection:

I read on DX-World that the 3DA0 team are having trouble syncing their FT8. I had the same problem on a remote DXpedition and used Time Fudge to manually adjust the time and it worked very well. Recently I stumbled across JTSync, which sinks your PC clock based on averaging times on FT8 etc. Have not used it yet but might be worth giving some publicity for future DXpeditions without internet.

Publicity is duly given, Paul !