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".
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":
After March 2023, life will be back to normal.
It's important to distinguish category and function.
"After" is a preposition so "after March 2023" can only belong to the category preposition phrase.
The PP is functioning as a modifier in clause structure, more specifically an adjunct, so we have:
Category: preposition phrase
Function: modifier (adjunct of time)
The same applies to your other example.
Note that the head word in a phrase determines its phrasal category (preposition phrase, adjective phrase, adverb phrase and so on).