Putting off an action leads to time wasting. If something is necessary, it is best to act quickly to accomplish it..
The English writer Edward Young, who coined this saying, published The Complaint: or, Night-Thoughts on Life, Death, & Immortality, more simply known as Night-Thoughts, in 1742. Although Young isn't as widely as contemporaries like Pope and Samuel Johnson, he was revered by them and Johnson called him ‘was a man of genius and a poet’. The Night-Thoughts poem itself is a major work and has been described as the 18th century's greatest long poem. Long is unarguable; it consists of nearly 10,000 lines of blank verse. It is in nine sections - the ‘Nights’ of the title and was published in serial form between 1742 and 1746.
The poem involves a nocturnal speaker grieving over the deaths of a child, wife and a friend and finding consolation in Christian thoughts. The 'procrastination is the thief of time' line appears towards the beginning of the work:
Be wise to-day; ’tis madness to defer;
Next day the fatal precedent will plead;
Thus on, till wisdom is push’d out of life.
Procrastination is the thief of time;
Year after year it steals, till all are fled,
And to the mercies of a moment leaves
The vast concerns of an eternal scene.
A similar thought, published in a work that would have been known to Young is found in Robert Greene's Gwydonius, 1584:
You shall finde that delaie breedes daunger, & that procrastination in perils is but the mother of mishap.