“Every moment lost is worth the life of a thousand men.”

Source: The Professor at the Breakfast Table (1859), Ch. X.
Context: Poets are never young, in one sense. Their delicate ear hears the far-off whispers of eternity, which coarser souls must travel towards for scores of years before their dull sense is touched by them. A moment's insight is sometimes worth a life's experience.

Innkeeper's wife
A Child is Born (1942)
Context: Life is not lost by dying! Life is lost
Minute by minute, day by dragging day,
In all the thousand, small, uncaring ways,
The smooth appeasing compromises of time,
Which are King Herod and King Herod's men,
Always and always. Life can be
Lost without vision but not lost by death,
Lost by not caring, willing, going on
Beyond the ragged edge of fortitude
To something more — something no man has ever seen.
Context: Life is not lost by dying! Life is lost
Minute by minute, day by dragging day,
In all the thousand, small, uncaring ways,
The smooth appeasing compromises of time,
Which are King Herod and King Herod's men,
Always and always. Life can be
Lost without vision but not lost by death,
Lost by not caring, willing, going on
Beyond the ragged edge of fortitude
To something more — something no man has ever seen.
You who love money, you who love yourself,
You who love bitterness, and I who loved
and lost and thought I could not love again,
And all the people of this little town,
Rise up! The loves we had were not enough.
Something is loosed to change the shaken world,
And with it we must change!

Variant: every moment you steal from the present is a moment you have lost for ever. There's only now.
Source: The Passion

John Rivers in The Genius and the Goddess (1955)
Context: You can't worship a spirit in spirit, unless you do it now. Wallowing in the past may be good literature. As wisdom, it's hopeless. Time Regained is Paradise Lost, and Time Lost is Paradise Regained. Let the dead bury their dead. If you want to live at every moment as it presents itself, you've got to die to every other moment.

Dijkstra (EWD1239: A first exploration of effective reasoning)
1990s

Canto VI, stanza 18.
The Lady of the Lake http://www.gutenberg.org/etext/3011 (1810)

Very often attributed to Addison, this is apparently a paraphrase of a statement by Hugh Blair, published in Blair's Sermons (1815), Vol. 1, p. 219, where he mentions "men of pleasure and the men of business", and that "To the former every moment appears to be lost, which partakes not of the vivacity of amusement".
Misattributed

Epigraph, Ch. 2 : Twenty Carats Fine.
Shadows of Shasta (1881)
Context: A thousand miles of mighty wood
Where thunder-storms stride fire-shod;
A thousand flowers every rod,
A stately tree on every rood;
Ten thousand leaves on every tree,
And each a miracle to me;
And yet there be men who question God!

The Younger Brother, Act III, sc. ii (published posthumously 1696).