Both of them are about the mother's son from the future time traveling to the past to help his mom get together with his dad. Both of them also made their presence known by introducing themselves to the mom.

The difference is in I Belonged to Your World, the person the son wants to set up his mom with isn't his birth dad (he wants his mom to live a better life by not marrying his birth dad) and the son is already much older. In Please Be Married, the person the son wants to set up his mom with is his birth dad and the son is much younger.

Both main leads have a mutual crush/love that lasts from high school until graduation. The main leads are also in the same friendship circle, which started because they were deskmates. In both dramas, the male lead is smarter than the female lead and helps her out in her studies. Both of them had problems during their university time.