Arrch, which part are you saying is incorrect?
To my mind,
time now - time earlier = elapsed time
0x00000020 - 0xFFFFFF20 = 0x00000100
You may get the wrong result if the calculation is made after the time has rolled over for 24+ days or something on the extremes:
But normally it will be correct.
I would expect "ride time" in this case might be on a per trip basis, or added & stored in EEPROM, but millis() would be reset when the engine was shutdown and later restarted.