This formula should work, when everything is OK... Two possible causes:
1. The field name "date" is misspelled - should it be capitalized, like Date ? Field names are case sensitive.
2. The field "date" is not a date but a string (depends on how it's loaded from the source data). In this case, it needs to be converted into a proper date field before the Month() function can be used.
If you could share a bit more about your data, we could possibly troubleshoot your issue.
Oleg - wouldn't the wrong field name spit out an error message on the load?
Gregory - To Oleg's second point - you might first want to identify exactly how QlikSense is consuming your date field. Let's say the format of the date field = 02_03_15.. or MM_DD_YY. You could then 'force' QS to read as a Date with the Date# function:
Date#(Date,'MM_DD_YY') <-- This tells QS how to read the date. You can then nest that function with other date functions in order to achieve your desired result:
Date(Date#(Date,'MM_DD_YY'),'MM/DD/YYYY') <-- Date() is a formatting function.