When does the 30 day free time that comes with the preorder begin?
Straight after the head start?
Including the head start?
When you first log on when the full game is released?

May 1st is when your 30 days begin.

I'm pretty sure that the 30 free days starts at launch.

Usually it will start when you first log in after head start.

head start is extra they won;t charge you for it just like they don't charge your time for obt starts on may 1st.

usually they start your free time the moment the game launches officially.
so a person who pre-ordered the game gets to play from the 28th of April.
but your 'free' subscription time will only start counting the 3rd of May when the game officially launches. so you get 5 days for free and then 37 days for free (until the 10th of june or something)

that's at least how most MMO's do it (like swtor). I don't exactly know if tera will follow the same example or start your subscription the 28th (so you only get 32 official days extra).