/**
    * Calculates the difference between two angles.
    *
    * @param   a   The first operand. (a-b)
    * @param   b   The second operand. (a-b)
    * @return      The difference.
    */
private float DeltaAngle(float a, float b) {
    float diff = a - b;
    diff = diff % (Mathf.PI * 2);
    if (diff > Mathf.PI)
        diff = -Mathf.PI * 2 + diff;
    return diff;
}

float rot = DeltaAngle(desiredRotation, -(Mathf.Deg2Rad * GetComponent<Transform>().eulerAngles.y) - Mathf.PI / 2);
if (Mathf.Abs(rot) >= rotationSpeed * Time.deltaTime)
    rot = Mathf.Sign(rot) * rotationSpeed;
else
    rot = rot / Time.deltaTime;