I think this calculator might be off on it's math.  If you put in 1.15 as the growth rate, you get to the monthly increase in difficulty in 2 increases, but this is 28*.85 days, right?  There should have been a third difficulty increase for the last part of the first month.

Difficulty Start => 1690895.8030524
Growth => 1.15

2236209 = 1.15 * 1.15 * 1690895.8030524, but this is only 23.8 days.  Is my math off?

1   28.62   400.68   2236209.69954
2   20.37   285.18   3400995.42678
3   14.23   199.22   4497816.45192
4   10.13   141.82   6840616.59631
5   7.07   98.98   9046715.44863
6   5.04   70.56   13758923.3579
7   3.52   49.28   18196176.1409
8   2.5   35   27674109.3882
9   1.75   24.5   36599009.6659
10   1.24   17.36   55662518.8257
11   0.87   12.18   73613681.147
12   0.62   8.68   111957207.314