I looked in the man page for chage and the -E option needs a date or a number of days after January 2, 1970. So that would be a pretty big number. Try using a -e 2008-12-11 to give them 2 weeks.
-E, --expiredate EXPIRE_DATE
Set the date or number of days since January 1, 1970 on which the
userīs account will no longer be accessible. The date may also be
expressed in the format YYYY-MM-DD (or the format more commonly
used in your area). A user whose account is locked must contact the
system administrator before being able to use the system again.
Passing the number -1 as the EXPIRE_DATE will remove an account