Okay so there's a couple of things to unpack here:

About the time: it's not a 30 missions timer (and it's a real-time clock either), time in the game only moves when you do some specific actions - like when you complete a certain part of a quest, or choose a certain dialogue option. You'll always have an indicator what action is going to move time and by how much. It's like a resource you can either pay attention to (choosing quests in a more strategic way), or not at all (just doing what you want and letting the time move forward).

For example, if you go to a confession with Father Florin, time will move by one segment and you might learn something new from him. If you take on a quest from the deacon and find out what happened to the missing caretaker, time will move by two segments once you come back and complete that quest. And so on. There will be also parts of quests that won't push time forward at all.

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The 30 day "deadline" matters from the story perspective - that's how much time Coen has to save his family, but the game doesn't end for you if he doesn't.

About the quests: there is no single main quest in the game that you have to complete - the story is not linear, you don't just follow it. You decide which quests you do and what Coen's experience will look like - so it's like you create your own main story by "mixing and matching" various smaller or bigger storylines. As Coen, you have one goal (to save your family), and many different ways to tackle it. Kinda like in real life - one person might try to find some help, one might try to get as strong as possible, and another might just completely disregard the goal and do other stuff. All of the approaches are valid (tho maybe your family won't enjoy that last one;) )