Hi Sako,
Using AllDates as you have described, causes increasing Time part (hour) of DateTime. To increase Year only, you should do:
exiftool -AllDates+="1:: ::" dir
-I'm not sure if double quotes must be used on Mac (if not, try single quite instead).
Actually, I like this more:
exiftool -AllDates+="1:0:0 0:0:0" dir
-because it's more descriptive which part will be changed.
Greetings,
Bogdan