When selecting grinding goal date, the system calculates how many battles per day you have to play.
The system calculates minimum 1 battle per day, it is hard to play half an battle ^_^
If the system calculates that you need 10 more battles to reach the goal, but you set the goal to be in 20 days, it will only be needed to play for 10 days until you reach the goal. Today the system revert the goal date according to remaining battles to play.
This will be changes, as it is a bit confusing. From version 0.23.2 the date you pick as goal will be shown and saved as the goal date.
Another example: if i set the goal 3 month ahed in time, but I only need 10 more battles to play, the system anyway show the goal date 3 month ahed in time. The number of battles and days remaing will show 10 anyway.