In romantic relationships, Instant Messaging (IM) can serve as a communication channel to maintain a sense of mutual presence and relational closeness when being physically separated. However, IM is asynchronous by design. There can exist time delay for people to receive and reply to incoming messages, which may violate romantic partner's mutual expectation. Limited understanding is available around how unintended and intended delays affect the relationship of romantic partners. This work examines how romantic partners grow, perceive, and use mutual knowledge about each other in delayed IM to resolve the expectancy violation. We conducted a 7-day diary study on 16 pairs of romantic couples and used the diary entries as probes for post-study one-on-one interviews. Our findings show that couples employ different strategies of information grounding to parse and resolve delayed IM. Based on these findings, we propose several theoretical and practical implications.