I’m not sure the exact date if one is given in the manga but in Sailor Moon R the movie we find a very young Usagi talking to Mamoru shortly after his accident. Now that event happens on his 7th birthday and since Mamoru was born 8/03/75 that means the crash took place on 8/03/81. Shingo’s age is 10 in the 1st story arcs of both the manga and the anime so it could be on a date in August but in the scene the bandages on his head are gone meaning he’s immediate injuries from the crash have healed and he’s being kept for observation at the time. During which at the same hospital Ikuko goes into labor and gives birth to Shingo. Even if we are not given the date would the month of August be correct birth month for Shingo?