In just three lines, the poem sets up a quiet scene: it is morning, the speaker is on the road, and both of these things push them to write letters. The writing doesn't seem casual, though. The last line turns it into something heavier, calling the letters "an urn for the last days."
An urn holds ashes, so the image suggests that the letters are a way of keeping what remains of a person's final days, perhaps the speaker's own, perhaps someone else's. The poem doesn't say which, and that openness is part of its pull. Judging by the title, "Time capsule," the letters are also meant to be sent forward, preserved for someone who will read them later.
The mood is calm but shadowed. Morning and travel usually suggest beginnings and movement, yet here they lead to thoughts of endings. The speaker seems reflective rather than panicked, treating writing as a gentle way to gather and hold onto time before it slips away.