since both effects belong to you
would this be difference if it were an opponents jar?? Ques not right?.
If it were the opponent's jar, then it would go on the stack in Active Player, Non-Active Player (APNAP) order.
So, at the end of your turn, Skullcap goes on the stack, then Jar. Jar resolves first, returning your old hand. The Cap resolves, causing you to discard it.