Both series focus on the undetermined nature of the female lead's relationship to the male lead. Suiyobi 22-ji dake no kare is the much more PG version of a undefined relationship, with arguably a time limit on the couple being 'together'. I also just really feel like both series have a similar vibe to them as well.
Hazuki, working for stationary manufacturer for a year, dates Satoru, held back at university. Things hasnt been going well between Hazuki and Satoru, and Hazuki starts dating Yasuda whom she met at the matchmaking party. On a Valentines day, Hazuki hands a chocolate to Yasuda which meant that he was her most favorite. In return, Yasuda gave her ear rings on a White day. But after receiving a present from Yasuda, Hazuki realizes something very important. When she opened her post after coming back from a date, there was a present from Satoru