Why do Argentines write their 9s backwards? I've noticed a lot of people do this and it bothers me so much more than it should...
I actually think the numbers may have been inverted - I know it's not customary, but I seriously doubt he was trying to give you a 1 day pass. Unless he specifically told you something? If they frown upon Colonia runs (which it seems they do) then you'd expect them to say something. If he wrote the date and made no mention of it, then he may have just been trying to write it American style so you could understand or made a mistake. Either way, I wouldn't worry about it. Worst case scenario you pay the 300 peso fine when you leave.