generally,没差 in chinese means there is no different in both of them,such like 这饭太难吃了,吃,没吃都没差.means this food is too terrible,ther is no different wheather i ate it.
but in Jay's song 没差 has a little different,becasue this song uses time difference to show a guy's feeling about a girl.so 没差 is a abbreviation of 没有时差,means no time difference.
i love you no time differene,everytime.