When the poem first revs up, we have a very clear idea of past and present. The speaker is clearly talking about how we can feel numb in the present after something horrible happened in the past. It's cool how the poem starts off with so much definition, but then gets blurrier as it moves on.
The stiff Heart questions 'was it He, that bore,' And 'Yesterday, or Centuries before'? (3-4)
These lines make things blurry in lots of ways. First, it's unclear if the "He" that's referred to is the Heart, itself, or Jesus, or maybe even some other person the speaker knows but doesn't bother to name. On top of that, the speaker seems unsure if the traumatic thing happened yesterday or a super long time ago. If whatever she's talking about happened "Centuries before," then it's pretty unlikely that it happened to the speaker herself. Could she be referring to some event in all of humanity's past? Could this line be digging into the whole idea of collective memory somehow?
This is the Hour of Lead – Remembered, if outlived, As Freezing persons, recollect the Snow – (10-12)
We've got two words in these lines that directly reference memory: "Remembered" in line 10 and "recollect" in line 12. What's weird about these lines is how they dislocate us in time. The first is pretty straightforward and references how we remember some period of numbness if we manage to make it through without dying. It gets messier with the second one, though. If a person were currently "Freezing," then why would they even need to "recollect the snow"? Wouldn't it still be snowing? It seems like these lines purposely blur the lines between past and present to get across the total disorientation that happens when we're still reeling from some painful event.