Can't use dynamic values or binding for "the part after the =>'" in animations. According to Chris Oliver, this is a Timeline implementation bug. In the example below the animation will always start at 10 instead of 50 at the 2nd start:

package e;

import javafx.application.*;
import javafx.scene.paint.*;
import javafx.scene.geometry.*;
import javafx.input.*;
import javafx.scene.*;
import javafx.scene.effect.*;
import javafx.scene.image.*;
import javafx.animation.*;
import javafx.scene.text.*;
import javafx.ext.swing.*;
import java.lang.*;

var dynX=10;
var x=10 on replace { System.out.println(x); };

var anim1 = Timeline {
     keyFrames : [
         KeyFrame {
             time: 0s
             values: [
x => dynX
// x => bind dynX //won't work
     ]
         },
         KeyFrame{
             time: 4s
             values: [
x => dynX*2 tween Interpolator.LINEAR
// x => bind dynX*2 tween Interpolator.LINEAR // won't work
]
         }
     ]
}

System.out.println("start1: "+x);
anim1.start();
System.out.println("end1: "+x);

Thread.sleep(5000);
dynX=50;

System.out.println("start2: "+x);
anim1.start();
System.out.println("end2: "+x);



///////////////

Will give the following output:
10
start1: 10
end1: 10
start2: 10
end2: 10
11
12
13
14
15
16
17
18
19
20