to make things as not confusing as possible, my rule of thumb is:

• yyyy-mm-dd (yyyy instead of yy ensures that it's not mistaken for dd-mm-yy) (hyphens can be replaced with underscores)
• dd.mm.yyyy (yyyy same as above) (really dislike using for filenames, sorting doesn't work)
• mm/dd/yyyy (only if there is no other choice) edit: mm/dd/yyyy vs mm/dd/yy doesn't matter because both make 0 sense already edit2: i forgor to say that yyyy also avoids y2.1k and subsequent issues