I have set time by hand in the field using the GPS in my phone as the time source, with good results for FT8. If you don't have a phone with GPS, a shortwave receiver should be able to receive correct time, either from one of the national time beacons, or by listening to FT8 signals. The FT8 software shows you the time difference between each decoded message and your local computer clock. You can use this to verify that your clock is close to what the crowd believes to be the correct time. You don't need perfection, but getting to within a half second should be possible and should work well.