Comma before ‘before’
You don’t normally need a comma before a time clause. You need it after the time clause if it comes before the main clause. This is the standard rule with most secondary clauses, and it’s a reasonable, practical rule:
The kids wanted to go to the pool after we ate…
After we ate, the kids wanted to go to the pool…
In the first case, the conjunction makes it clear where the clause begins. In the second example, put a comma to show where the secondary clause ends and the main clause begins, or the reader may have the temporary problem of interpreting ‘after we ate the kids’. Or often you wouldn’t know where an adverb belongs.
However, in your case (17/20) the comma is not absolutely superfluous as you have a longish place adverb before the time adverbial. It’s a good pause, it gives the reader the opportunity to see things in their proper perspective.
Leave ‘at’.
In 17b I’d get rid of ‘of course’ altogether. If you keep it, keep the comma with it.
You don’t normally need a comma before a time clause. You need it after the time clause if it comes before the main clause. This is the standard rule with most secondary clauses, and it’s a reasonable, practical rule:
The kids wanted to go to the pool after we ate…
After we ate, the kids wanted to go to the pool…
In the first case, the conjunction makes it clear where the clause begins. In the second example, put a comma to show where the secondary clause ends and the main clause begins, or the reader may have the temporary problem of interpreting ‘after we ate the kids’. Or often you wouldn’t know where an adverb belongs.
However, in your case (17/20) the comma is not absolutely superfluous as you have a longish place adverb before the time adverbial. It’s a good pause, it gives the reader the opportunity to see things in their proper perspective.
Leave ‘at’.
In 17b I’d get rid of ‘of course’ altogether. If you keep it, keep the comma with it.