It is now possible to use date formaters in log filenames (in mask options).
For example to have log by year and month in separate directories, you can do :
/set logger.file.mask "%Y/%m/$plugin.$name.weechatlog"
You'll have a tree like that:
~/.weechat/
|- logs/
|- 2009/
|- 12/
| irc.server.freenode.weechatlog
| irc.freenode.#weechat.weechatlog
|- 2010/
|- 01/
| irc.server.freenode.weechatlog
| irc.freenode.#weechat.weechatlog
