By today; before the end of the day — an adverbial phrase indicating a deadline within today.
そのレポートは今日中に提出してください。
Please submit that report by the end of today.
今日中に見積もりを出せるように手配します。
I'll arrange to have the estimate ready today.
忙しいけれど、今日中になんとか終わらせます。
I'm busy, but I'll somehow finish it by today.