A less dirty workaround to get accurate clip start and end times

LessDirtyClipStartEndTimes.cs

//[Omitted...]
public class CustomTransformationTrack : TrackAsset
{
	public override Playable CreateTrackMixer(PlayableGraph graph, GameObject go, int inputCount)
	{
		var mixer = ScriptPlayable<CustomTransformationMixerBehaviour>.Create (graph, inputCount);
		
		foreach (TimelineClip clip in GetClips())
		{
			var customClip = clip.asset as CustomTransformationClip;
			if (customClip != null)
				{ customClip.CustomClipReference = clip; }
		}
		return mixer;
    	}
}

//[Omitted...]
public class CustomTransformationClip : PlayableAsset, ITimelineClipAsset
{
	public CustomTransformationBehaviour template = new CustomTransformationBehaviour ();

	public TimelineClip CustomClipReference { get; set; }
	
	//[Omitted...]
	public override Playable CreatePlayable (PlayableGraph graph, GameObject owner)
	{
		var playable = ScriptPlayable<CustomTransformationBehaviour>.Create (graph, template);
		playable.GetBehaviour().CustomClipReference = CustomClipReference;
		return playable;
	}
}

//[Omitted...]
public class CustomTransformationBehaviour : PlayableBehaviour
{
	public CustomTransformation transformation;
	public float angle = 90f;
	public Vector3 axis = Vector3.right;
	public bool reverse;

	public TimelineClip CustomClipReference { get; set; }
	public double CustomClipStart { get { return CustomClipReference.start; } }
	public double CustomClipEnd { get { return CustomClipReference.end; } }
}

//[Omitted...]
public class CustomTransformationMixerBehaviour : PlayableBehaviour
{
	private bool _firstFrameHappened;
	private Quaternion _defaultRotation;
	private Transform trackBinding;
	
	public override void ProcessFrame(Playable playable, FrameData info, object playerData)
	{
		trackBinding = playerData as Transform;

		if (!trackBinding)
			{ return; }

		int inputCount = playable.GetInputCount();
		if (inputCount == 0)
			{ return; }

		if (!_firstFrameHappened)
		{
			_defaultRotation = trackBinding.localRotation;
			_firstFrameHappened = true;
		}
		
		double playTime = playable.GetGraph().GetRootPlayable(0).GetTime();
		Quaternion rotation = Quaternion.identity;

		for (int i = 0; i < inputCount; i++)
		{
			var inputPlayable = (ScriptPlayable<CustomTransformationBehaviour>)playable.GetInput(i);
			CustomTransformationBehaviour input = inputPlayable.GetBehaviour();
			
			if (input.transformation == null)
				{ continue; }

			double customDuration = input.CustomClipEnd - input.CustomClipStart;
			float normalizedClipPosition = Mathf.Clamp01((float)((playTime - input.CustomClipStart) / customDuration));
			float eval = input.transformation.Evaluate(input.reverse ? 1 - normalizedClipPosition : normalizedClipPosition);
			Vector3 rotationToAdd = input.axis * (input.angle * eval);

			rotation *= Quaternion.Euler(rotationToAdd);
		}

		trackBinding.localRotation = _defaultRotation * rotation;
	}

	public override void OnGraphStop(Playable playable)
	{
		if (trackBinding != null)
			{ trackBinding.localRotation = _defaultRotation; }

		base.OnGraphStop(playable);
	}
}