Is it possible that Yuni got the date wrong, and Nanase got the date right, and hence make the call right after midnight?
That's a good guess. Personally, I think it's just Nanase forgot their anniversary because the time Nanase called Yuni was 00:52. On their previous anniversary, Nanase always called right at 00:00.
Still, even though she got the wrong date I can feel Yuni's pain. I mean, they are in a secret relationship. Showing even a little intimacy in public is difficult. For me, it's like Nanase doesn't give her enough like security about their relationship? Sort of that. That's why Yuni always get upset whenever, especially here, her best friend (maybe? Not sure) gets clingy to her. You know, that scene during festival when Yuni saw her sharing some food with the other girl really hurts. So I understand why she cheated with Nanase. But the moment she kissed Fuuko, she should break-up with Nanase.