I wonder if They bore it long—Or did it just begin— (5-6)
Grief is a mystery, and time is an aspect of this mystery. Grief doesn't seem to have an end, which is a majorly depressing thought if you dwell on it too long.
I could not tell the Date of Mine—It feels so old a pain—(7-8)
Not only is the grief of others a mystery, so is the speaker's own grief. On the one hand, the speaker's using time to express the degree of her sadness—she's been sad for so long, she can't even remember when it began. But on the other hand, she is also being coy with us; it's almost as if she's saying, "Oh, I could never tell you that!" Perhaps the date would be too revealing, perhaps it would be easy to trace back the reasons for her sadness if she dated it for us. Throughout the poem, we get several reasons why other people in the world are sad, but we never know why the speaker is sad. It's just one of those Dickinson mysteries.
I wonder if when Years have piled—Some Thousands—on the Harm—That hurt them early—such a lapseCould give them any Balm— (17-20)
Sometimes grief feels like it has gone on for thousands of years, even though this is impossible. The speaker might be suggesting that because it can seem to have existed forever, sadness rebels against logical ideas about time.