ベティーは、日本でホームステイをした時、スリッパの使い方になれるまで、ずいぶん時間がかかった。(from AIAIJ)
I was wondering why した時 is used. To me, this gives me the nuance of "After Betty did her homestay..." rather than the (I think) correct reading of "When Betty was doing her homestay..."
Is there a reason why this is preferred to using something like している時? The action that Betty does appears to take place as she is doing her homestay, not after, which would seem to not warrant the past tense preceding 時. Or perhaps there is a more crucial part of the sentence I misread?