I googled this question yesterday
I saw the question asked several time , with dozens of responses , but not actually answered the question , all the answers were why , , but not when
but from what I could gather
back in WWII , the US military still used the month/day/year , they didn't change to the day/month/year format to well after the war
but I'm not positive
but then , it could have also been the way he wrote his numbers , I noticed that also , at work we have a guy in my department form the Ukraine , and he rights his numbers all funky , their kinda hard to read
but I don't know if that would be enough to give someone away as a spy , this was the late 30's , early 40's , even people in the US were still isolated , even here in Iowa and Minnesota , 80 percent of the population had German ancestors , and alot of one room school house , so many of those children still had German roots , with German-ish teachers
but what seemed weird , is , if it was the way he wrote his numbers or the date , wouldn't that be one of the first things they would teach a German spy
you would think that would be taught in German spying 101