Use the File list block to get an array of all file paths in the directory, then start a For loop of that array. Use the File exists block to get the last modified timestamp of each file, then use an Expression true block with the formula "modifiedAt < (Now - time(24 * 15))". If it's true, then the file is older than 15 days and you can connect the YES dot to a File delete block. Otherwise, the NO dot can connect back to the IN dot of the For block.