Not sure if you still need help, but in the absence of any other reply I'll offer what I have found.
It appears the times are random, not sure how random, not sure if it can be all times or just some from a select group.
But the way to tell is to open the casket to goto the screen that gives you the option to try to open it, right click the image and save the url ( or whatever is your operating systems equivalent)
you will get a link like this
http://warofdragons.com/images/data/misc/sky_time_v.gif
the thing to look for is the very last letter before the .gif, in my case v
a= 0-1, b= 1-2 , c= 2-3 etc
v= 21-22
thats 12 hours from now for me, so will let you know how I go.