Fletch gave you the answer you were looking for -- you need to add 1 to $mon, just like you add 1900 to $year. But I wondered if you might rather want your string to be:

    return sprintf( "%d_%2.2d_%2.2d", $year, $mon, $mday );
                # e.g. "2005_09_16"
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(update: original "join" approach would have produced "2005_9_16", which drives me nuts.)

That way, a standard ascii-betic sort of file names would put them in chronological order (assuming the date string is the first part that differs from one file name to the next).

I suppose some people prefer to see a sorted list that puts things together according to the month they were created, regardless of the year, but I've never understood people like that.