I tried to filter budget and set the http request header (‘If-Modified-Since’) with a future datetime. I still get all budget rather than the empty. Can anybody help me with the issue?

var client = new HttpClient();
client.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer",     config.AccessToken);
client.DefaultRequestHeaders.Add("xero-tenant-id", config.TenantID);

client.DefaultRequestHeaders.IfModifiedSince = DateTime.Now.AddDays(2);

var response = client.GetAsync(uri).Result;
if (response.IsSuccessStatusCode)
{
    content = response.Content.ReadAsStringAsync().Result;
}

I tried to filter budget and set the http request header (‘If-Modified-Since’) with a future datetime. I still get all budget rather than the empty.