I'm not sure what happened, but it seems some of my diamond mines that had been exhausted came back (I think?). Now two of them are due to expire on the same day, at the same time, and three more are due to expire exactly 24 hours after them at the very same time.
if you're basing that on the fact they expire on thesame day, you're not totally correct, as the mines durations are chosen randomly.
The mine duration depends on the output of resorces of the mine. You can have week or strong mine. I have found the location for the good mines even though they play out faster I have better returns. A week mine will last longer.
I have noticed that they play out after they same amount of resorces have been reached. I think total amount is the key...random my play a part but I don't see it. You should do the math.