By looking at the decompiled code of the game, the profession works just like fertilizers. The whole number of days is reduced by 10% then rounded down, which mean the reduction will always be at least one day.
For multi-harvest crops, the time between the harvest is not affected. Only the initial grow time (before the first harvest) is reduced.
Each single crop will be affected individually, which mean they will all receive the exact same reduction and will all be available for harvest the same day. For multi-harvest crops will have regular harvest.
This mean that cauliflowers will grow in 10 days, and parsnips in only 3 days!
Relevant code is available here. Look at lines 234 to 240, tho it might be hard to understand as the decompiler does not preserve variables name.