Make sure your date format is correct. The fact that the problem occurs after the 12th day makes me think that the day /month/ year format you're using is the problem. It's based on American date format and you may have entered Canadian format. Just a thought.
The calendar on the "Report a new cache" side is delivering the format dd/mm/yyyy and the side is not accepting this.
-> This is definitly a bug and should be fixed. Does nobody checks the new features if they are working in European Mode before delivering it?
By "European Mode", do you mean "Almost-everywhere-in-the-world-apart-from-America Mode"?
yes, but I'm not sure about the Date-format in e.g. the UK. But I also found out right now that even the Metric system is not fully supported - On the Hide&seek pages it still askes for the radius in Miles.
However, when looking around on the side (I did not take a nearer view on that topic so far), I found out that all dates (e.g. the renewal date or the found dates in your personal list and even the Hidden date on a cache) are in US-style date (which is the most illogical one of all, but this my personal view), but a Event is switched to "Event Date : Monday, 14 June 2010" and as I said before, the page wanted me to enter it as dd/mm/yyyy
This is completely illogical and it is a very bad design of the user interface, because it would completely confuse the user. But when looking at the side, I think there is at all no design-masterplan.
I think, it should be possible to select your date-format like you select your unit-format (metric or imperial), and that must be independent from the language you choose.