Time is the process in which Stardew Valley proceeds. Each day is 24 hours, and each hour is sixty minutes. Time passes in ten minute intervals. Each interval equates to about seven seconds in real-time. There are 7 days per week. An hour of the game consumes 42 seconds of real-time. From that, we can assume that a full game-day duration is about 17 minutes.
At in-game twelve o'clock, your player will become tired. If you are still out of your home at two a.m., you will stop whatever you are doing and fall asleep. You wake up the next morning in your bed, with a note in your mailbox that a neighbor pulled you to your bed after seeing someone rob your unconscious body. You lose a few hundred dollars.
The world becomes dark at nighttime. If a light source disappears at nighttime, nearby NPCs will run away. The inside of houses will need to be lit.
There are four seasons each year: Spring, summer, fall, and winter.