Could be an IJN ensign, it has the off-centered sun and should have 16 rays. Black light might help, but it looks pretty new and even if it is pre-45, could have been washed with modern detergents.... I'd say wartime if original, they used this all the way back to 1870... but like everything else, has been reproduced..... I'm new to Japanese flags but so far I see no major red flags (no pun intended) with this piece... best of luck,
Mike