Convert rotation from sign switching to continuous in Animation Nodes Plane is simply rotating on Z axis around origin from 0 deg to infinity (linear interpolation), AN extracts this rotation from polygon. Extracted motion looks fine but it isn't calculated as cumulated/continuous value. Every 180 degrees it is switching sign from + to -.

Look at Z value here:



Can this rotation be recalculated to match input motion? I was planning to change some values inside material in [0-1] range using growing angle, but unable to do it with current behaviour.
This signed representation is called a Directed Angle. To convert that to a non-directed angle, simply add $2\pi$ if the result is negative as follows:



This is assuming you angles are in radians, if they are in degrees, you should be adding 360 instead.