What happens when you run the command by hand? Does it work? If so, it could be a permissions issue. Check all files involved to make sure that the cron user has privilege to read and/or execute where applicable.
If it runs manually, then I'd guess it's got something to do with how things are chmod'd and/or chown'd. Find out what user cron runs as, and check to see if the mongo binary can be run by it. Also, check the permissions and ownership of 15days.js