I can't do it today because you just told me about it today.
I can't help today because you didn't give me any notice until now.
ーI can't do it today because you just told me about it today.
「今日言われて今日はできません。」
「今日できない」は I can't do it today.
You just told me about it today で「あなたはそれについて今日言ったばかりです。」
ーI can't help today because you didn't give me any notice until now.
「今まで私に何にも知らせてくれなかったので、今日は手伝えません。」
「今日できない」は I can't help today とも言えます。
you didn't give me any notice until now で「あなたは今まで私に何も知らせてくれなかった」
ご参考まで！
Sorry, I don’t have time to do it because I’m busy today.
Sorry, I don’t have time to do it because I’m busy today.
「ごめんなさい、今日は忙しくてそれをやる時間はありません」
You should have told me yesterday.
「昨日言ってくれればよかったのに」
You should have told me earlier.
「もっと早く言ってくれればよかったのに」
ご参考まで
カナダ 
日本 
