God has promised forgiveness to your repentance; He has not promised tomorrow to your procrastination.
Attr. to St. Augustine.
I Googled, and of course found the usual “quote” sites. I checked Google Books, and this shows up in Fr. Paul Scalia’s That Nothing May be Lost. The preview even showed a citation! How exciting! And then I checked … and it cites a secondary source.
Google Books did turn up Flowers of Christian Wisdom from 1873, which gives it … without citation.
So I went to the handy Everything St. Augustine site (my name for it) and searched for promisit, which is Latin for “has promised.” It returned 298 hits. None of them was this saying.
But wait! A reader intervened! See his comment below. It appears that someone took ideas from one or more of St. Augustine’s homilies and condensed them into this saying. Thank you, O reader!
As I’ve said before, it is a brave man who will claim to be sure that something is nowhere in the voluminous works of St. Augustine, and I’m not that brave. But I’m very skeptical.
Verdict: Unproven and unlikely. Heavily paraphrased and summarized.
They didn't say it 