There is the Animation - Casting point (ucpt) value on unit types. It is used on most spells unit's of this type cast and is done before any spell effect triggers, also you can cancel meanwhile.
@Bribe: machris13 means the casting time field of the spell, which generally locks the unit when starting to cast until it is over. After this time, it will check if the target is still in range. If it is not, it wont trigger the spell's effects but instead the unit will try to approach the target once more, it is unlocked at this time again but will start the casting time anew when the target is acquired.