Where is the line between a crime of passion and pre-meditated murder?

I was thinking about what constitutes pre-meditated murder today and it got me to thinking that part of this has to be time. If someone buys a gun with the intent of killing his boss a week before he kills his boss, I would consider that pre-meditated. However, if he buys the gun a half hour before he kills his boss, is that a crime of passion or is it pre-meditated? The way I see it, if he has a drive to work that’s a half hour long, that seems like enough time to realize that he shouldn’t go kill his boss. However, if he’s angry enough to actually consider killing his boss, then a half hour certainly isn’t enough time to cool down. What if this time was extended? An hour? Two hours? A whole day? Also, how does time affect preparation for murder? If he buys a gun, bleach, a tarp and a saw and then goes to kill his boss, is it pre-meditated if getting the supplies and killing his boss all takes less than an hour?