There’s no timeline component in widgets.
If this must be a timeline, have it play in the player and feed the widget appropriate, resulting data. I assume it’s supposed to be an animated fade with a very specific timing, right? Because that’s what timeline are for.
If the fade lasting X seconds is not a requirement, you could interpolate on Tick instead, and let the fade arrive at the desired value. You can control the speed of the interpolation.
Widgets kind of do have a timeline in the form of animation, have a look here for how to make it work with lerp:
Essentially, you play an animation and treat its progress as alpha. The feature is not that well documented, though.