While a real-time clock allows for a number of new possibilities such as Christmas motifs on December 25th, a number of games choose to rely on it in order to gate progress or unlockables, many times disguised in the form of an in-game puzzle. When utilized in single player games, it can typically be seen as a cause for frustration for players looking to complete a game, as no amount of effort or intense play can bypass these hard-coded gameplay appointments. A player has the option to either wait for the event, or move on. Many choose the third option.
Note that this concept generally wouldn't apply to online games such as MMOs, as in those cases the scheduled content is designed to encourage players to sign on to the service and congregate in major hubs.
Adjusting the System Clock
In situations where the gated content is required for completion percentage or achievements, many players resort to manipulation of the system clock in order to avoid waiting the hours, days, or months necessary for the game event to occur legitimately. This issue was anticipated for the Animal Crossing series, and the developers even included fourth wall-breaking character Mr. Resetti as a mild punitive measure in order to discourage the behavior so players could enjoy the games as intended.
Recently, this can be seen in the extreme with the Storyteller achievement in Batman: Arkham City. The game was released on October 18th, 2011 with an achievement that required players boot up the game and visit the Calendar Man in his cell on and around every major holiday, as indicated on the calendar days circled adjacent to the cell. For a dedicated player purchasing on day one, the first day the achievement would be legitimately possible would be on Labor Day, September 3, 2012, nearly a year later. Instead, the first recorded instance of the achievement being unlocked on Giant Bomb was on October 23, 2011, only five days after the game was released.
More recently, Fez was released with one collectible required for two achievements being completely unreachable outside of a small window opening every two days at 12PM EST.