Ahistorical means outside of history, and that's the feeling the poem builds. The speaker describes time as "thin as pain," as if the hours have worn down to something fragile and easy to feel through. Yesterday doesn't weigh much anymore. It has shrunk, and it looks "pretty," which suggests the past has been softened and smoothed over, perhaps a little falsely.
In the second half, the "we" steps in and gets busy. They make up "a bespoke weather of the hour," a mood or climate tailored to the present moment, and they "find things / to own and talk to." That last phrase is both funny and a little sad. People turn to objects for company and ownership when the larger sweep of time feels slight.
The mood is quiet, wry and slightly lonely. The poem doesn't complain. It simply watches how we build small, private comforts when the past fades and the present feels thin.