Sam4: Rav Moshe says R”T is preferred but changes what R”T said because it cannot be that there’s pitch blackness and it’s not night yet.
Old man: I’ve heard that explanation before. It’s probably P’shat. But once again, I have to be very hesitant before saying anything definitive on this subject. Also, calling a Mil 18 minutes really only works if you assume the SH”A is sometimes talking in fixed hours. 24 works much better (in just about every Rishon) and is very clear from the Be’ur Hagra (I forget which Siman but it’s a S’if 2; it’s the one where the SH”A talks about Shkia), assuming that it’s Zmaniyos and when the SH”A says 3/4 Mil he means 18 Zmaniyos minutes. Rabbi Dr. David Shabtai had an article about this in one of the YU Torah publications (maybe there’s a Kol Tzvi on Shabbos or Bein Hashmashos or something) and his P’shat really seems the most likely.