This is really freaking adorable because Chibi Usa is wanting to use her TIME KEY to TRAVEL THROUGH TIME to go back to her home in the future and Usagi’s like “it’s too dangerous to travel at night.” Day and night have no meaning where she’s going. In fact she’s literally traveling through the Underworld and it’s like 100x more dangerous than a dark alley in Tokyo could ever be.
And you know what? I don’t think Usagi’s forgotten that. I think she’s just really happy to have an excuse to hang out with Chibi Usa for an extra night. And Mamoru and Chibi Usa are both more than happy to go along with it because they all want the same thing.