Change to theim-cfg-base directory.
See Instant Messaging Server Directory Structure for information on locating im-cfg-base.
See iim.conf File Syntax for instructions on locating and modifying iim.conf.
Add the following value for iim_arch.chat.categoryname:
iim_arch.chat.categoryname = Archive:Chat:%Y:%M
The archive provider automatically assigns the current year for %Y and current month for %M. These values are taken from the system date and time.