Is this the correct usage of a comma?
The parties were living together until 3/24/20, which is when Janet moved out.
Is the comma used correctly here?
Solution 1:
Yes, it's used correctly.
which is when Janet moved out is a supplementary relative, not an integrated, as it doesn't help to define 3/24/20.
The comma works to separate it prosodically from the rest of the sentence indicating a slight pause in speech.
In writing, the comma serves to mark the fact that the relative is not a modifier of the noun phrase it takes as its antecedent, but a supplement.