I do not know if this is intentional, but i noticed that moonlight detectors will work at day if their covered with opaque blocks. a way to fix it is to have the sensor check for game time every several ticks(2-8), then output the corresponding redstone strength.
Environment
windows 7 pro 64-bit; service pack 1, java 7; update 65; standard edition; 64-bit. specs: CPU; Intel core 2 duo P8600 @ 2.4 GHz GPU; Intel Mobile 4 series express chip-set family. 2 gig ram (1.90 usable) 150 gig hard drive(hdd).lenovo laptop.
THis is the same as the covered daylight detector working at night: MC-5927