You don't want to be sending anything within the
send() call since this is a GET request. That would be alright if you were POSTing the data, but for a GET you just append the data to the URL ("time.php?name=...").
Code:
var url = "time.php?name="
+ document.myForm.username.value
+ "&password="
+ document.myForm.password.value;
xmlHttp.open("GET", url, true);
xmlHttp.send(null);