实体框架选择上个月记录

我有员工工资表，其中包含：

public partial class S_EmployeeSalary
    {
        public int SalaryId { get; set; }
        public int TypeId { get; set; }
        public int UserId { get; set; }
        public double Salary { get; set; }
        public Nullable<double> ExtraSalary { get; set; }
        public Nullable<double> Insurance { get; set; }
        public Nullable<double> Sanctions { get; set; }
        public System.DateTime SalaryDate { get; set; }

        public virtual C_UserItems C_UserItems { get; set; }
    }

我有一个按钮，当用户单击它时，它会自动创建上个月记录的副本，以更新创建的新月份的记录：

public ActionResult CreateNewRows(int typeId)
{
    IQueryable<S_EmployeeSalary> moduleItems = db.S_EmployeeSalary
        .Include(x => x.C_UserItems)
        .Where(x => x.TypeId == typeId && DbFunctions.DiffDays(x.SalaryDate, DateTime.Now) > 30 && DbFunctions.DiffDays(x.SalaryDate, DateTime.Now) < 30);

    foreach (var item in moduleItems)
    {
        S_EmployeeSalary entity = new S_EmployeeSalary
        {
            TypeId = typeId,
            UserId = item.UserId,
            Salary = item.Salary,
            ExtraSalary = item.ExtraSalary,
            Insurance = item.Insurance,
            Sanctions = item.Sanctions,
            SalaryDate = DateTime.Now
        };
        db.S_EmployeeSalary.Add(entity);
    }

    db.SaveChanges();
}

我的问题是我如何只指定上个月的记录来选择它，我尝试了 DbFunctions.DiffDays 但效果不佳，有没有人可以帮助我获得新想法？ 记下其员工的工资，所以我需要上个月而不是最近30天

取决于您想要上个月还是过去 30 天（问题不清楚）

这是上个月的情况：

var startOfTthisMonth = new DateTime(DateTime.Today.Year, DateTime.Today.Month, 1);       
var firstDay = startOfTthisMonth.AddMonths(-1);
var lastDay = startOfTthisMonth.AddDays(-1);

IQueryable<S_EmployeeSalary> moduleItems = db.S_EmployeeSalary
    .Include(x => x.C_UserItems)
    .Where(x => x.TypeId == typeId && 
                x.SalaryDate >= firstDay &&
                x.SalaryDate <= lastDay);

这是过去 30 天的数据：

var firstDay = DateTime.Today.AddDays(-30);

IQueryable<S_EmployeeSalary> moduleItems = db.S_EmployeeSalary
    .Include(x => x.C_UserItems)
    .Where(x => x.TypeId == typeId && 
                x.SalaryDate >= firstDay);