Which unit has the difference of two Date.now() calls?

If you like to check the runtime of you code you can use something like this:

var t1, t2;
t1 = Date.now()
//begin code to messure
a = analogRead(D28);
// end code to messuere
t2 = Date.now()
print("runtime:", t2-t1);
/* output
runtime: 0.88500976562
*/

Are this 0.885 mili seconds or 0.885 micro second

Reference say:

Get the number of milliseconds elapsed since 1970 (or on embedded platforms, since startu)

So the difference should be micro seconds. Not shure because a output of Date.now() looks like this 41866529.26635742187.