The other answers escape the “g” in “log” since “g” is a special character in datePattern. This isn’t wrong, but I prefer to wrap the entire set of non-date characters in single quotes, like so:
<datePattern value="yyyy-MM-dd'.log'" />
This gives the same results, but is easier for me to manage. This way, I don’t have to recall which specific characters are special for datePattern (the list is long and varied). If I forget one character then I don’t run the risk of borking my file names; they’re all nicely escaped en masse.