[I wasn't intending this post to get this wordy so feel free to pretend it's not even here]
So I went back through all the chapters so I could piece together a timeline of what's gone on. Mostly for myself, but maybe others can find it useful as well. The key part to remember is that because of the release schedule, even though chapters have been uploaded since last June, there have only been 15 of them so far, and even in those we haven't even gone through a full week (2.5 mostly covers the events from 1.2 from Nanami's side, with a few pages at the start and end that are focused on her only).
.>Monday after school the punishment game is decided. Baron challenges Youshin to make Nanami fall for him. [chapter 1.1]
.>Tuesday after school is the confession, bucket hit + infirmary, walk home, exchange contacts, and go to school next day. Baron challenges to hold hands by the end of the week. (this is one part that comes off as off, since Youshin responds that there are "only two days left," which only works if you ignore Wednesday entirely) [1.2 & 2]
.>Wednesday is walking to school, holding hands/arms, announcement of dating to class (dear gob is that a breath of fresh air contrasting to so many "hide our relationship" stories), lunch together, after school date to buy a bento box, and date set up for Sunday (coming from Baron's challenge for Saturday). Nanami decides that she's going to make Youshin fall for her. [3, 4, 4.5]
.>Thursday's two main defining events are the brief conversation about Nanami's aversions to guys and the basketball challenge with Shibetsu-senpai (Youshin's win!). Nanami wants to chu his cheek. [5, 6, 7, and the start of 7.5]
.>Friday has Senpai wanting to be friends, and being turned down on joining the basketball club. Also, lots and lots of Nanami affections. [the rest of 7.5 and the starting pages of 8]
.>Saturday is going to the mall and running into Hatsumi, Ayumi, and a mysterrrrious third person -- oh wait, it was Nanami. [8 & 9]
.>Sunday brings us to now with the movie date main event, making gyoza together, then walking her home gently late at night into meeting the Barato family [10 & 11]
Even taking into account the story being more from Youshin's side than Nanami's, he seems to run the "it's just a month long punishment game" thought far more than she does. And while she
does acknowledge it, it's not a solitary emotion.
I think by the time of the date she's pretty decided that "one month" no longer matters, even if she's still aware of it being present.
But between that interaction and her father saying she lied to them happening on the same day, she's probably worried that she has been or will be found out before he goal of making him fall for her comes to be (it's only been 4 days since she decided that goal).