2 Peter 3:8 - Matthew Poole's English Annotations on the Holy Bible

Be not ignorant of this one thing; i.e. be sure of it: the same word is here used as 2 Peter 3:5; and so he cautions them against the ignorance of scoffers, and to prevent it, would have them certainly know this one thing, which is extant in the Scripture, which foretells Christ's coming. That one day is with the Lord; the Lord Jesus Christ, of whose coming he speaks. As a thousand years; by a synecdoche, a thousand years is put for any, even the longest revolution of time; and the sense is, that though there be great difference of time, long and short, with us, who are subject to time, and are measured by it; yet with Him who is eternal, without succession, to whom nothing is past, nothing future, but all things present, there is no difference of time, none long, none short, but a thousand years, nay, all the time that hath run out since the creation of the world, is but as a day; and we are not to judge of the Lord's delay in coming by our own sense, but by God's eternity.