Detect file changes in node.js via watchFile

I want to detect changes for a file, if the file changes, I will use child_process to execute a scp command to copy the file to a server.I looked up node.js documentation, the fs.watchFile function seems do what I want to do, but when I tried it, somehow it just doesn't work as I expected. The following code were used:

var fs = require('fs');                                                                        

console.log("Watching .bash_profile");

fs.watchFile('/home/test/.bash_profile', function(curr,prev) {
    console.log("current mtime: " +curr.mtime);
    console.log("previous mtime: "+prev.mtime);
    if (curr.mtime == prev.mtime) {
        console.log("mtime equal");
    } else {
        console.log("mtime not equal");
    }   
});

With above code, if I access the watched file, the callback function get execute, it will output the same mtime, and always output "mtime not equal" (I only accessing the file). Outputs:

Watching .bash_profile
current mtime: Mon Sep 27 2010 18:41:27 GMT+0100 (BST)
previous mtime: Mon Sep 27 2010 18:41:27 GMT+0100 (BST)
mtime not equal

Anybody know why the if statement failed(also tried using === identify check, but still get the same output) when the two mtime are the same?