The Chronos' Timer enables your horses to train twice as fast, and therefore complete their training at a young age. Rides, training, and skills earned through competitions are the activities affected by Chronos' Timer.
Be aware that Chronos' Timer will not double the skills your horse can acquire. It will only enable its learning speed to double.
When used with Water Of Youth, Chronos' Timer will still work, but is effectively cancelled out. It will take the same amount of effort to produce the same results, but the horse will only be half the age.