Defective Key Word
You: second and third quatrains, second line of couplet, capitalized to characterize and emphasize whom I am addressing as a very relevant person to me, capitalization almost makes “You” a name, distinguishes abilities of this individual from those of Time in context of last lines of third quatrain and couplet
Key Word
not: meant to emphasize the infinite difference between what could have and what did not happen or what can and what will not happen

Deliberate digression from iambic pentameter in line 12, meant to stress unique idea that a person may provide something that time alone won’t.

I like the idea of anything else about the poem being left to the imagination of the reader