by today; before the day is over
Used to indicate that something must be done or will happen within the current day, before midnight.
Please submit this document by today.
I hope the rain stops before the day is over.
Compound of 今日 (きょう, 'today') + 中 (じゅう, 'within') + に (adverbial particle). The reading じゅう is a regular rendaku form of ちゅう.