You will want to convert your partial date text-string to a SAS numeric variable DATE value (days since 1/1/1960). In a DATA step, you can take your partial string and concatenate a "01" to complete your date string in format "yyyymmdd", and use the ANYDTDTE. or another SAS date-related INFORMAT to derive your SAS numeric DATE variable.
Given your post, perform this task for each of your two partial-date text-string data values, and then use the INTCK function to derive a variable that will represent the months between your two dates.
The SAS support website
http://support.sas.com/ has SAS product documentation and technical (user community, SUGI/SGF) papers on this topic. I have provided one example below for your reference.
Scott Barry
SBBWorks, Inc.
Using SAS® Dates and Times – A Tutorial
Jonas V. Bilenas, JP Morgan Chase, Wilmington, DE
http://www2.sas.com/proceedings/forum2007/226-2007.pdf
SAS DOC main page:
http://support.sas.com/documentation/onlinedoc/base/index.html
SAS DOC on INFORMATs by category:
http://support.sas.com/documentation/cdl/en/lrdict/59540/HTML/default/a001239776.htm
Sample doc item on SAS dates/times:
http://support.sas.com/publishing/pubcat/chaps/59411.pdf