Makes sure each animation tick has the same timestamp for all animations in the batch.

src/effects.js

@@ -11,7 +11,17 @@ var elemdisplay = {},
 		[ "width", "marginLeft", "marginRight", "paddingLeft", "paddingRight" ],
 		// opacity animations
 		[ "opacity" ]
-	];
+	],
+	fxNow;
+
+function clearFxNow() {
+	fxNow = undefined;
+}
+
+function createFxNow() {
+	setTimeout( clearFxNow, 0 );
+	return ( fxNow = jQuery.now() );
+}
 
 jQuery.fn.extend({
 	show: function( speed, easing, callback ) {
@@ -349,7 +359,7 @@ jQuery.fx.prototype = {
 		var self = this,
 			fx = jQuery.fx;
 
-		this.startTime = jQuery.now();
+		this.startTime = fxNow || createFxNow();
 		this.start = from;
 		this.end = to;
 		this.unit = unit || this.unit || ( jQuery.cssNumber[ this.prop ] ? "" : "px" );
@@ -394,7 +404,8 @@ jQuery.fx.prototype = {
 
 	// Each step of an animation
 	step: function( gotoEnd ) {
-		var t = jQuery.now(), done = true;
+		var t = fxNow || createFxNow(),
+			done = true;
 
 		if ( gotoEnd || t >= this.options.duration + this.startTime ) {
 			this.now = this.end;