计算某一日期是在一年中第几周
var cld = System.Globalization.CultureInfo.CurrentCulture.Calendar;
cld.GetWeekOfYear(dt, CalendarWeekRule.FirstDay, DayOfWeek.Sunday);
var cld = System.Globalization.CultureInfo.CurrentCulture.Calendar;
cld.GetWeekOfYear(dt, CalendarWeekRule.FirstDay, DayOfWeek.Sunday);