根据日期计算出周一和周日

DateTime baseDate = DateTime.Today;

var today = baseDate;
var yesterday = baseDate.AddDays(-1);
var thisWeekStart = baseDate.AddDays(-(int)baseDate.DayOfWeek);
var thisWeekEnd = thisWeekStart.AddDays(7).AddSeconds(-1);
var lastWeekStart = thisWeekStart.AddDays(-7);
var lastWeekEnd = thisWeekStart.AddSeconds(-1);
var thisMonthStart = baseDate.AddDays(1 - baseDate.Day);
var thisMonthEnd = thisMonthStart.AddMonths(1).AddSeconds(-1);
var lastMonthStart = thisMonthStart.AddMonths(-1);
var lastMonthEnd = thisMonthStart.AddSeconds(-1);

DevExpress的横轴，根据时间，自适应

周一为每周的第一天，进行时间的推断

private static DayOfWeek firstDayOfWeek = DayOfWeek.Monday;

public static DayOfWeek FirstDayOfWeek
{
get { return firstDayOfWeek;}
set { firstDayOfWeek = value; }
}

public static DateTime GetWeekStart(this DateTime time)
{
DayOfWeek lastDayOfWeek;
if (firstDayOfWeek == DayOfWeek.Sunday)
{
lastDayOfWeek = DayOfWeek.Saturday;
}
else
{
lastDayOfWeek = firstDayOfWeek - 1;
}

DateTime weekStart;
if (time.DayOfWeek == lastDayOfWeek)
{
//用默认的第一天减去最后一天,相差6天
weekStart = time.AddDays(DayOfWeek.Sunday - DayOfWeek.Saturday);
}
else
{
weekStart = time.AddDays(firstDayOfWeek - time.DayOfWeek);
}
return weekStart;
}