I agree with what others have said about why memory can be hazy about dates and details.

I'd also like to add to that that in the world these guys live in, days and months really blend together because their lives aren't structured around dates the way the average person is. These guys are living scheme to scheme, constantly dealing with beefs, sitdowns and the daily hassles of being a criminal. One day they may have to meet someone at 10 am, the next day they're off doing something at 3 am. A lot of standard time just blends together. They'll remember things around seasons, whether it happened in summer or around christmas because those backdrops are ubiquitous (if it's summer, you can't avoid the weather telling you that it is summer. at christmas there's christmas decorations everywhere in the city). Their lives are not structured around common time, so I think it's understandable they often don't have dates. Most of the books the only time you get actual dates is from court records and police notes. It's very rare that the gangsters actually recall exactly when something happened.