//今天
DateTime.Now.Date.ToString();
//昨天
DateTime.Now.AddDays(-1).ToString();
//明天
DateTime.Now.AddDays(1).ToString();

//本週第1天及最後1天 (要注意的是這裡的每一週是從週日始至週六止)
DateTime.Today.AddDays (-(int)DateTime.Today.DayOfWeek).ToString();
DateTime.Today.AddDays (6 -(int)DateTime.Today.DayOfWeek).ToString();

//上週, 一個周是7天,上週就是本週再減去7天
DateTime.Today.AddDays (-(int)DateTime.Today.DayOfWeek - 7).ToString();
DateTime.Today.AddDays (6 -(int)DateTime.Today.DayOfWeek - 7).ToString();

//下周
DateTime.Today.AddDays (-(int)DateTime.Today.DayOfWeek + 7).ToString();
DateTime.Today.AddDays (6 -(int)DateTime.Today.DayOfWeek + 7).ToString();


//巧用C#裡ToString的字符格式化
//本用第1天及最後1天
DateTime.Now.ToString("yyyy-MM-01");
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).AddDays(-1).ToString();

//上個月第1天及最後1天
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(-1).ToString();
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddDays(-1).ToString();
//下個月第1天及最後1天
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(1).ToString();
DateTime.Parse(DateTime.Now.ToString("yyyy-MM-01")).AddMonths(2).AddDays(-1).ToString();

//今年第1天
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).ToString();
//今年最後1天
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).AddDays(-1).ToString();

//去年第1天及最後1天
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(-1).ToString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddDays(-1).ToString();

//明年第1天及最後1天
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(1).ToString();
DateTime.Parse(DateTime.Now.ToString("yyyy-01-01")).AddYears(2).AddDays(-1).ToString();

//本季,一年四個季度,一個季度三個月
//先把日期推到本季度第一個月，然後這個月的第一天就是本季度的第一天了
DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
//本季度的最後一天就是下季度的第一天減一
DateTime.Parse(DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToString();
//下季
DateTime.Now.AddMonths(3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
DateTime.Parse(DateTime.Now.AddMonths(6 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToString();
//上季
DateTime.Now.AddMonths(-3 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01");
DateTime.Parse(DateTime.Now.AddMonths(0 - ((DateTime.Now.Month - 1) % 3)).ToString("yyyy-MM-01")).AddDays(-1).ToString();