httpmessagecontent in mvc api

使用方法httpmessagecontent时如何返回datetime（我正在建立一个接收并返回android工作室的网站。）这是我的功能代码：

[HttpGet]
public HttpMessageContent TurnDetails(string ServiceName)
{
    int a = db.Sessions.Where(k => k.Service.Description == ServiceName).Count();
    var r = db.Services.Where(l => l.Description == ServiceName).Select((l => l.SessionLength));
    var f = db.Services.Where(k => k.Description == ServiceName).Select(k => k.Openinghours.Select(p => p.OpenTime));
    a = a * Convert.ToInt32(r) + Convert.ToInt32(f);
    Convert.ToDateTime(a);
    return a;
}

return Convert.ToDateTime(a);