If you have multiple games on your battle.net account does it take days from all of them

Each game that is listed on your Battle.net account has its own subscription (if any). This means that if you have two games, with different amounts of subscription days on them, you can end up with different subscription end dates.

Each subscription gets days taken separately, so you have to pay for both subscriptions. Merging them in a signle batt.enet account does not change that.