Other than for keeping track of gameplay time or dating save files with the last time saved, what games use a real-time clock and calendar? Nearly all that I know of are Nintendo's, and most of them I own... Now why aren't there more games like these?
Extensive use:
Animal Crossing, Animal Crossing: Wild World
Pokémon: Silver/Gold/Crystal/Sapphire/Ruby/Emerald Versions
Day/Night:
Mario Party 6 (certain mini-games, and I think whole boards are different in day and night)
Star Wars: Rogue Leader - Rogue Squadron II (that one mission where you steal that Imperial shuttle)
Very minor:
Marvel Vs. Capcom 2: New Age Of Heroes (the big clock in one background shows the current time...yay?)
Rise Of The Triad (in this old PC game, I recall that Christmas put Santa hats on the main characters and changed the music to a remix of "Rest Ye Merry Gentlemen")
Anything else?