//月初肯定是一号
DateTime.Now.ToString("yyyy-MM-01");//本月初
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).AddDays(-1).ToShortDateString();//本月最后一天
//可以将输入的年月替换下
string starttime=DateTime.Now.ToString("2017-12-01");//本月初
string endtime = DateTime.Parse(DateTime.Now.ToString("2017-12-01")).AddMonths(1).AddDays(-1).ToShortDateString();//本月最后一天
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