You need to work out the -mtime +<days> and -mtime -<days>
The +<days> = older than, -<days> = younger than
Code:
find /path/to/files -mtime -30 -mtime +13 -print | \
while read file
do
cp "$file" /new/path/`basename "$file"`
done
You may want to mess around with just the first line, until it prints the correct files for you.
