About six years ago I wrote such a program in the Quickbasic language for the DOS system (which predated Windows). It calculated the the lat and lon of the midpoint of the grid square. I think I also took care of the irregularities in some of the quadrants. It should be correct for the North and South Atlantic and maybe even the Mediterranean. I tested it extensively for the grids covering the Gulf of Mexico and the Caribbean Sea.
(I didn't extend it to the Pacific because I needed to know the coordinates of the borders of the quadrants, but I didn't have a detailed map to go by.)
I don't know if anyone knows how to use DOS and Quickbasic anymore.