Yes, but exactly how depends on a few things. Is this string a part of a record contained in a file, and thus you want to replace all of these strings with just the last two chracters? Or, is this string in a ksh or bash script variable, and you'd like to truncate the string.
If it's the latter (you have the value in a script variable), there are better ways than using sed. For instance:
date="2012-12-01"
echo "${date##*-}" # deletes all characters up to, and including the last dash
If you have a file, then the syntax of each record of the file needs to be known in order to craft a sed programme that will do the work.