You should use the stat function :
According to the documentation :
fs.stat(path, [callback])
Asynchronous stat(2). The callback gets two arguments (err, stats) where stats is a fs.Stats object. It looks like this:
{ dev: 2049
, ino: 305352
, mode: 16877
, nlink: 12
, uid: 1000
, gid: 1000
, rdev: 0
, size: 4096
, blksize: 4096
, blocks: 8
, atime: '2009-06-29T11:11:55Z'
, mtime: '2009-06-29T11:11:40Z'
, ctime: '2009-06-29T11:11:40Z'
}
As you can see, the mtime is the last modified time.