//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(); [Page]
//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();