I am looking for a simple formula x(t) to calculate the position of a planet.
Simple means, the planet is moving on a two dimensional plane following the curve of an ellipse.
My problem is not the curve of an ellipse. Its to calculate at what time the planet is at what position.
All formulas I found are to complicated to understand.
Edit:
I want to use the formula for creating a two dimensional visualisation of the solarsystem. Objects, for example planets, should rotate around a central mass. The curve should be a ellipse and the system sould be stable.
If you want an approximate solution, you may use ellipses for the orbits of planets and other objects that orbit the Sun in closed paths.
We further simplify our representation and put every object in the same plane. And, as a final simplification, we will consider that the major axis of the ellipse is parallel to the $x$ axis.
Since the all objects will have the Sun in one of the focus of the ellipse, the Sun can be placed at $(0,0)$. Using this, the equation of each object can be written as: $$ x(t) = a \cos (2 \pi t / T + \theta) + \epsilon a $$ $$ y(t) = a \sqrt{1-\epsilon^2} \sin (2 \pi t / T + \theta) $$ where $T$ is the period of that object, $a$ is the semi-major axis of the ellipse, $\epsilon$ the eccentricity of the orbit, $\phi$ the angle between the major axis of the ellipse and the $x$ axis and $\theta$ the initial phase angle.
If you want to simulate the solar system, you can use the semi-major axis $a$ (in Astronomical Units, AU), the period $T$ (in years, yr) and the eccentricity (dimensionless) e.g. using the data from this site.
If you try to "simulate" some solar system objects with this equations you'll find that the planets are quite far apart from each other and that the periods are rather different.
In case you want a more accurate model, you should consider that the major axis are are not aligned for all planets (so, we would need to apply a rotation matrix to $(x,y)$. Furthermore, since the orbits are not coplanar, we should consider a 3-dimensional model and rotate each orbit plane according to its inclination to the Earth's orbit (or some other plane of reference).
Have fun!