Celestia doesn't explicitly include that particular functionality, but there are various ways to accomplish something similar.
For example, you ccould write Lua code to produce that result. Unfortunately, I don't think anybody has done it and published the procedure.
Here's an outline of one way it could be done:
One of Celestia's Lua functions is "getposition", which returns the coordinates of an object at a specific simulation time. See https://en.wikibooks.org/wiki/Celestia/Celx_Scrip ... ethods/Celx_object#getposition
An SSC object can be positioned using a Lua function as a ScriptedOrbit. See https://en.wikibooks.org/wiki/Celestia/Trajectories#ScriptedOrbit
An SSC object can be defined to act as the marker. Its SSC definition would have to include a ScriptedOrbit definition which calls the getposition function to place the "marker" object at the appropriate position.
There probably are easier ways to accomplish whatever it is that you actually want to do.
1. write a Cel or CelX script which alternates Celestia's simulation date between two (or more) values.
2. have two copies of Celestia open, one set at the current date and one set at the other date.
3. define multiple SSC "Earths", each at a different location (perhaps by specifying different MeanAnomaly values) along its orbit. You'd have to define them using an EllipticalOrbit specification which approximates the Earth's orbit instead of using the built-in CustomOrbit.
I'm sure there are others I haven't thought of.