Stardew Valley is a peaceful farming game in the same vein as Animal Crossing or Harvest Moon. Unlike these games, Stardew Valley runs on its own calendar rather than using a real life equivalent.
Time in Stardew Valley is broken up into four seasons, with 28 days each. Each day has 24 hours, but a second in our time is equivalent to about a minute in Stardew Valley time. While this time difference is most likely for gameplay purposes, I'd like to discuss Stardew Valley as if the game took place on another planet.
A year, one full revolution around our sun, would take about 44.8 hours in our time. Due to the exactness of the seasons in the game, the orbit of the planet would have to be nearly circular.