It is currently January.
For every 1000 experience points, a character receives one of the following bonuses:
30% +10 Health
25% +1 Attack
20% +10 to a random stat
15% +1 Defense
10% random perk
Completing one quest usually takes one month. Additionally, even if the characters decline the quest after embarking on it, they won't be able to accept a new quest in the same month.