How to calculate angle of a sector of an ellipse given its area?

Question

I'm creating a spreadsheet tool that will calculate the rough distance between planets in a fictional solar system. I know the various orbital eccentricities, periods, and semimajor and semiminor axis for each. For simplicity I'm assuming the orbits share the same major axis plane. With the periods and area of the ellipses I can break down how much area is in a day's movement in the orbit. Other questions I've seen here have got me to the point where I can calculate the area of an ellipse given the angle, so my idea is to add x days of area to that initial area and use that total area to find the new angle.