After March 2023, life will be back to normal.
Is the bit "After March 2020" an adverbial clause or a prepositional clause?
After eight hours, they reached the peak.
Same doubt about "After eight hours".
It’s a prepositional phrase that adverbially modifies what ensues.
You’re assuming it’s one or the other, but that’s a false assumption as prepositional phrases are modifying phrases, either adjectival or adverbial, and it just so happens that it’s adverbial in your sentence. As such, it is both a prepositional phrase and an adverbial phrase there.
What it isn’t is a subordinate clause, not adverbial or any other kind. In order to be a subordinate clause, it would have to contain a verb. It doesn’t, so it’s not.
For further information, you may refer to the following link and scroll down to the heading "Understand what prepositional phrases do in a sentence":
Source : Link , Question Author : res , Answer Author : Benjamin Harman