float2 pivot = float2(0.5, 0.5);
// Rotation Matrix
float cosAngle = cos(_Time.x * _Rotation);
float sinAngle = sin(_Time.x * _Rotation);
float2x2 rot = float2x2(cosAngle, -sinAngle, sinAngle, cosAngle);
// Rotation consedering pivot
float2 uv2 = i.uv - pivot;
i.uv = mul(rot, uv2);
i.uv += pivot;
i.uv = clamp(i.uv, 0, 1);
If you plan on doing effects like this I strongly suggest just rotation the game object instead.