I have a timelapse using the physical sun, it looks great, my only issue is I want to ease in the last keyframe -- so after the sun sets, we slowly stop under the stars -- yet I have no option to do so. In the key properties drop down I'm offered only step and linear. I pull up f-curves and there isn't a curve to play with.
add a new Special Track/Time to the Physical Sky (in the Timeline) - that makes it the second Time track in your physical sky so call it something else (TimeControl?). Then in the original Time track drag TimeControl into the Time Track slot in the Properties.
The new Time Track (TimeControl if you named it that) does have an F-Curve you can play with