LINQ中不能直接使用DateTime，否则会报错：‘The specified type member 'Date' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are support’
需要把DateTime定义在LINQ之外，例如：
var now = DateTime.Now.Date;
var query = from a in _workPeriodRepository.Table
where a.StartDate <= now && a.EndDate >= now
select a;
return query.ToList();