there's no interval while tilling the coarse dirt into farmland

step 1:create a world and place a coarse dirt
step 2:take out a hoe to till the coarse dirt with right click holding
step 3:take out a rooted dirt and do the same thing
conclusion: you will find that there's an interval between tilling the rooted dirt into dirt and tilling the dirt ingot farmland, but it won't happen on coarse dirt.