In 1.8, the inverted daylight sensor was a perfect inversion of the daylight sensor; when the daylight sensor was at 0 strength, the inverted was at 15. When the daylight sensor was at 1 strength, the inverted was at 14, etc. However, in 1.9 the inverted daylight sensor cycle seems to operate completely independently from the regular cycle. This is not mentioned in the changelogs, so I'm assuming it's a bug.

To reproduce:

1. Create a superflat peaceful redstone world with day-night cycle turned off
2. set time to 18000
3. place an inverted daylight sensor and a line of redstone 15 blocks out

you'll notice that the power level is only 11, and not 15 like it used to be.