I do not like to use < or > on strings as I do not think that they evaluate in the way that you desire, especially on dates.
I am guessing that it appeared to work for you this past year because your data source had no records with any Jan dates of a 2011 or less.
To see how the "calendar" strings functions grab a DB with multiple years of data in it and convert the date to your "MM/dd/yyyy" string and sort on that string. You will see the sort not even close to a calendar day sort.
Add another formula as a using your > and < comparison and you will see the multiple years on those dates be included.
In general, if you want to do date comparisons try to use the actual date field type or convert them to YYYYMMDD