Timeline can go negative.

lib/animation-timeline.js

@@ -2762,7 +2762,9 @@ var timeline_Timeline = /*#__PURE__*/function (_TimelineEventsEmitte) {
     key: "_formatUnitsText",
     value: function _formatUnitsText(ms) {
       var isSeconds = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : false;
-      // 1- Convert to seconds:
+      var sign = TimelineUtils.sign(ms) < 0 ? '-' : '';
+      ms = Math.abs(ms); // 1- Convert to seconds:
+
       var seconds = ms / 1000;
 
       if (isSeconds) {
@@ -2805,7 +2807,7 @@ var timeline_Timeline = /*#__PURE__*/function (_TimelineEventsEmitte) {
         str += seconds;
       }
 
-      return str;
+      return sign + str;
     }
     /**
      * Left padding of the timeline.
@@ -3543,8 +3545,8 @@ var timeline_Timeline = /*#__PURE__*/function (_TimelineEventsEmitte) {
       var source = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : TimelineEventSource.Programmatically;
       val = Math.round(val);
 
-      if (val < 0) {
-        val = 0;
+      if (val < this._options.min) {
+        val = this._options.min;
       }
 
       if (this._val != val) {

src/timeline.ts

@@ -1217,6 +1217,8 @@ export class Timeline extends TimelineEventsEmitter {
    * @param isSeconds whether seconds are passed.
    */
   _formatUnitsText(ms: number, isSeconds = false): string {
+    const sign = TimelineUtils.sign(ms) < 0 ? '-' : '';
+    ms = Math.abs(ms);
     // 1- Convert to seconds:
     let seconds = ms / 1000;
     if (isSeconds) {
@@ -1257,7 +1259,7 @@ export class Timeline extends TimelineEventsEmitter {
       str += seconds;
     }
 
-    return str;
+    return sign + str;
   }
   /**
    * Left padding of the timeline.
@@ -1875,8 +1877,8 @@ export class Timeline extends TimelineEventsEmitter {
    */
   _setTimeInternal(val: number, source: TimelineEventSource = TimelineEventSource.Programmatically): boolean {
     val = Math.round(val);
-    if (val < 0) {
-      val = 0;
+    if (val < this._options.min) {
+      val = this._options.min;
     }
 
     if (this._val != val) {