September 17, 2015. Difficulty rose by +4.17% to 59.34B from 56.96B.
Change
+4.17%
New difficulty
59.34B
Period length
13.4 days
Network hashrate then (7-day)
0.4 EH/s
Bitcoin price then
$228
Hashprice then
$1,958/PH/day
The 2,016 blocks before it took 13.4 days instead of 14, so the network had more hashrate than the difficulty assumed, and difficulty rose by 4.17%: an ordinary adjustment. For every machine on the network that is a pay cut of the same size until the next retarget.