lambda如何实现这种group

ItemModel m1 = new ItemModel(1488029396000l, 1488032996000l);//startTime 2017-02-25 21:29:56 ——endTime 2017-02-25 22:29:56
ItemModel m2 = new ItemModel(1487995200000l, 1487998800000l);//startTime 2017-02-25 12:00:00 ——endTime 2017-02-25 13:00:00
ItemModel m3 = new ItemModel(1487908800000l, 1487916000000l);//startTime 2017-02-24 12:00:00 ——endTime 2017-02-24 14:00:00
/*我想得出一个map，key是startTime对应的那天0点0分的时间戳，value是endTime-startTime的值除以1000(即总秒数)

• 即得出 {1487865600000=3600 ,1487952000000=7200}
• */ List itemModelList = new ArrayList(); itemModelList.add(m1); itemModelList.add(m2); itemModelList.add(m3);
  

 DateTime GetTime(string timeStamp)  
{  
  DateTime dtStart = TimeZone.CurrentTimeZone.ToLocalTime(new DateTime(1970, 1, 1));  
  long lTime = long.Parse(timeStamp + "0000000");  
  TimeSpan toNow = new TimeSpan(lTime); return dtStart.Add(toNow);  
}  

list.ToDirectory(x => new DateTime(GetTime(x.第一个字段).Year, GetTime(x.第一个字段).Month, GetTime(x.第一个字段).Day), x => (第二个字段 - 第一个字段) / 1000);