Eternal Equinox

So, you're playing your favourite game. A rather addictive game. But you notice something weird. Even though you've been playing this game for months now (and you know that a similar amount of time has passed in-game), the day night cycles always trigger at the exact same time or, in other words, the length of night and day doesn't change. It's convenient certainly but this obviously doesn't happen in real life - there's a certain thing called seasons.

Congratulations! Your game's world uses an Eternal Equinox. This is when no matter what latitude or time of year it is, the lengths of night and day do not visibly change, no matter where you go or how much in-game time you spend.

However, notice this is not a prerequisite, nor does the game in question have to use real-time or clocks of any sort. All it has to do is to have the apparent lengths of day and night be constants.

If it's assumed that everything happens on equatorial latitudes, it can be considered Truth in Television. The length of the day on such places varies by much less than one hour throughout the year. This goes completely unnoticed by locals, unlike the actual climate variation, which is influenced by sea and wind currents.

Can be best explained by the Law of Conservation of Detail. It doesn't have anything to do with the game so you shouldn't think too much about it.

Examples of Eternal Equinox include:

Action-Adventure Games

Averted in The Legend of Zelda Majoras Mask, given that it uses a day/night cycle, but takes place over only a few days, so there wouldn't be as much noticeable variation in times for sunset and sunrise.
- Played straight, however, in Ocarina of Time, Wind Waker and Twilight Princess, which also have day and night. Skyward Sword has both day and night, but you never see the transition from one to the other, so we don't know if this is happening or not. The rest of the series takes place exclusively in daylight.
Castlevania II: Simon's Quest: "The morning sun has vanquished the horrible night." Even stranger, there is no twilight period. It goes straight from day to night or vice versa.

MMORPGs

World of Warcraft has day and night changing at the same time (corresponding with the real time based on the server's time zone), regardless of longitude or latitude. That is to say, the days are equally long in every part of the world regardless of the time of the year, and the sun rises all over the world simultaneously.
Averted in MUME (Multi Users in Middle Earth) - the length of day and night changes according to season.

Platformers

The Jak and Daxter series doesn't actually have clocks, but the sun rises and sets in pretty much the same interval no matter where you are.

Role-Playing Games

Pokémon Gold and Silver/Crystal introduced a built-in clock that had mornings start at 4am and night start at 6pm.
- This is changed in Pokémon Black and White. The different times of day start and end at times dependent on the season. For example, daytime lasts from 9 am to 7 pm in the summer, but in the winter it lasts from 11 am to 4 pm.

Simulation Games

Animal Crossing has sunset and sunrise always occuring at a set time, regardless of latitude or time of year.
The Sims games, despite being taking place over a fairly long time in in-game days, uses a fixed day/night cycle throughout.
- This happens even in The Sims 2 Seasons. Even when in this expansion the seasons changes -and player can choose the seasons desired- with heat, rain and snow, the Earth position doesn't seem to affect the fixed day/night cycle duration.
In every one of the Harvest Moon games, sunrise is 6 AM and sunset is 6 PM, regardless of season.
- Not so in IoH/SI. In those, the times of sunset vary depending on the season. Sunrise is less clear-cut.

Turn-Based Strategy

The Battle for Wesnoth does this. On the associated fora, it was first justified as the result of the granularity of the turns, but then pointed out that at it's apparent latitude and granularity, there should still be noticeable change, but kept as an Acceptable Break From Reality.
Almost-but-not-subverted in Hearts of Iron II. Day and night does vary with latitude, so days are shorter in the Northern Hemisphere. But they're always shorter in the North - apparently it's always winter there.

Wide Open Sandbox

The GTA games run on this trope - Every Real Life second is 1 in-game minute, night/day are always the same length.
Survival mode in Minecraft. A full day/night cycle lasts twenty minutes: ten minutes for day, seven minutes for night, and ninety seconds for sunrise and sunset. Since dangerous monsters come out at night, it's good that the passage of time is reliable day after day.

Non-Video Game Examples

The Truman Show was implied to have a constant, instant day/night cycle.
Truth in Television for places on the equator, where days are always the same length and seasons don't really exist.