For anyone who is interested here's a cognos date format bug for year patterns:
Using this format dd-MMM-YYYY (uppercase year pattern) for the dates from 29th Dec 2014 to 31 Dec 2014 the year will be displayed as 2015 instead of 2014. This happens just for presentation (HTML, PDF, Excel is fine). Programatically it works fine.
Changing the above format to dd-MMM-yyyy (notice the year pattern, lowercase) presents the correct year
Here's what IBM says on the documentation for date patterns:
"The case of letters used in pattern matters. For years, both y and Y are the same. For months, "MMM" is short name of month. "MMMM" is the full name of month. "mmm" displays month in numeric format. For days, "d" is the day of the month. "D" is the day of the year."
This issue happens in 10.1.1 and we haven't tested in other versions.
This happens with 10.1.1 and 10.2 as well. This issue has now been logged with IBM