There is only one reliable method to determine the time of harvest and that is to watch the 'trichomes'. Google it!
Get a magnifier that is strong enough so that you can see the color of these small stalked wonders.
I've never had a ill timed harvest since I started watching the 'trichomes' except for the time when I was forewarned of an impending search warrant on my place. Needless to say EVERYTHING got harvested and moved before they came busting in my front door.
It was still quite a rush but the look on their faces was worth the loss of a prime crop and I'm free to grow many more.
It's good to have friends in the right places!