The first statement excludes Jan and Feb because there are two dates only occur once because roy would know if it was 17 or 18. And since jos said roy doesnt know it isnt jan or feb.
After the 2nd statement there are only three possible outcomes march 15, dec 16 and dec 14.because for roy to know the dates should only occur once.
After the 3rd statement the only possible outcome is March 15 coz jos only knows the month and it couldn't have been dec coz it had two dates.
So March 15 is my answer