A year in Horse Reality takes 16 days in real life. Horses age 1 month every 32 real-life hours at the age rollover time (00:00 in-game time). This happens automatically (and is separate from the manual rollover) and cannot be slowed down or stopped, but it is possible to speed the ageing process up using DP.

Manual Daily Rollovers
Daily rollovers occur manually, meaning the player is responsible for starting the rollover whenever they first enter the game on a new day (00:00 in-game time starts a new day). This can only be done once per day and must be clicked before you are allowed to enter the game, which means that you can't accidentally miss it. After clicking the rollover button your horses will need to be cared for again and their physical fitness and affection will drop.

https://horsereality.wiki/en/World/World-Basics/Time

32h är en månad i spelet.
Ett år är 16 dagar

Tack!😁