From what I can tell the time starts as soon as you begin the first task of the day. Late last night, I started the first Tunnel 1 task about a half hour before starting the first task of Tunnel 2, and the times lined up that Tunnel 1's timer was set to expire a half hour before Tunnel 2's. Then early this morning (about two hours after EA's daily clock turns), I started the first Tunnel 3 task and it started counting down from 24 hours (meaning that the clock didn't start when EA's daily clock starts; otherwise it would've told me I had 22 hours or so left, not 24).
If I understand correctly, this is a combination of the two separate systems from the stonecutters.
In stonecutters, the "puzzle" prizes were unlocked every day, but we could still get them until the end of the event, just by solving the puzzles in sequence. That is, the prize did not become unavailable at the end of 24 hours.
In the stonecutters, in "daily tasks", we got our largest payout for our fourth task, so we tried to get all four before the 24-hour timer hit.
THIS event is such that the first two " tunnel prizes" that unlock each day do not disappear, we just have to do them in sequence before moving on to the next tunnel. Same as the "puzzle prize"s in Stonecutters.
However, the third prize is our "bonus" for getting the entire tunnel list completed within 24 hours.
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