Field the month of the event is invalid.

I get this error message when I try to enter the month in the field in my newly created content type. I have created two fields with datetime. One is to show the day (d) and the other is to show the month (M).
I've created two new date format types: 1) event day date format and 2) event month date format.
I've added two new formats: (d) and (M) respectively

1) In my day cck datetime field as "input format" for the day I have set it to 23 (today's date) as it was available from the list and as "default display" I've set it to "event day". The rest of the options are standard (granularity year, month, day, hour).

2) In my month cck datetime field as "input format" for the month I have set it to Feb (today's month) as it was available from the list and as "default display" I've set it to "event month". The rest of the options are standard (granularity year, month, day, hour).

Now, 1) works fine and can be stored but 2) gives this error: when I want to save "Field the month of the event is invalid."

I've overcome this problem by setting the "input format" for 2) to 23/02/2010 and the "default display" to "event month". That seems to work. Why is that? for 1) it works but not for 2).

If you know the answer, I'd love to hear it ;-)
BassPlaya.