Some variables don't need to be calculated real-time, those will include a lot of stuff, from deadlines to stuff like age and from how many players are in the game to how many gold you have in your bag. Even some variables that seem to be updated real-time are actually just updated when they need to be (see hit points example down below).
Calculate On Demand
Your gold, for example, is only ever updated when you spend it or get more added to it. The number of players in a server is only ever updated if a person joins or leaves the server.
The age of a player's character should only ever be updated if it is shown on the screen, in other words, when the player opens his character sheet. So, if a player wants to see his updated age, he will have to close and re-open the character sheet if it's already open.
Even stuff like hit points or mana need only ever be updated if they're changed. In games that do not have regeneration, it would be a waste to update the hit points every frame. Even in games that do have regeneration, it's still a waste to update it every frame, every second would be more than enough.
This is all to avoid heavy CPU usage.
Delegate To The Client
As an example, in a game like FarmVille, there are many timers and many times more players and it's impossible to do all the timers real time on the server side. So, the server sends when the timer will end to the player' computer (the client). The client will handle the calculations, and when the timer ends, it will send a request to the server and the server will either approve it or reject it. Usually rejection means the client's timer ended too soon and the server will most likely send a new timer value or delay the action if it's just a few seconds.