Returning claims from a token after authorization

TokenController.cs

[HttpGet]
[Authorize(Policy = "Certificate")]
[Route("Validate")]
public ActionResult<IEnumerable<string>> Validate()
{
  var token = "{";
  foreach (var claim in User.Claims)
  {
    //Datetimes are already escaped
    if (claim.Type.ToString().Contains("time"))
    {
      token += $"\"{claim.Type}\":{claim.Value},";
    }
    else
    {
      //Let's not care about the authentication method here
      //since that requires an array for building valid json
      if (claim.Type == ClaimTypes.AuthenticationMethod)
      { }
      else
      {
        token += $"\"{claim.Type}\":\"{claim.Value}\",";
      }
    }
  }
  //Remove the surplus comma
  token = token.Substring(0, token.Length - 1);
  token += "}";
 
  return Content(token, "application/json");
}