Strictly speaking,
Flight time means:

(1) Pilot time that commences when an aircraft moves under its own power for
the purpose of flight and ends when the aircraft comes to rest after
landing; or



Thus, neither Hobbs nor tach time are flight time. Hobbs time is likely to
be more accurate than tach time, however. I don't know how your digital
count-up timer works, but it is probably no more inaccurate than Hobbs time
would be. Or you could just use your watch, kitchen timer, GPS flight time
logger, or whatever else you have.