You can use a registry setting to do this.
See:
http://www.msgpluslive.net/help/registry/
quote:
\Preferences LogsDateDirFormat
Format used to create chat logs files or directories (depending on your chat logs settings and/or event log settings). This value can be used for cosmetic purposes or functional purposes. For example, a value of "yyyy" will force Messenger Plus! to create a new file/directory once a year and a value of "dd-MM-yy" will create a new file/directory once a day. See below for more details.