Another (easier) way could be:
let the sun rotate ‘around the earth’ at a constant speed (two Keyframes, Linear Interpolation) and in the time that the sun rotates 360°, let 24 hours pass (also at a constant speed). That would still leave the issue, that a Sun Light in Blender doesn’t have a Location…but with a Bone (or an Empty), a Point At and a Copy Rotation Constraint, that could be solved relatively easy, I think…
