获取去年同一周的同一天

Question

I was looking for a way to fetch the same day of the current week as a year ago. For example, today is:

• August 10th 2022 - Wednesday.

Assume this is the check-in date, the check-out date I expect to get is:

• August 11, 2021 - Wednesday.

Because it's the same day (Wednesday) as last year. But I need to take leap years into account, so I need to see if the current year is a leap year and if it is, if it has passed the 29th of February, the same with the date last year.

How to do this using .net core? I thought of something like:

private DateTime GetDayOneYearBefore()
    {

        if(DateTime.IsLeapYear(DateTime.Today.Year) && DateTime.Today.Month > 2){
            return DateTime.Today.AddDays(-365);
        }
        else if(DateTime.IsLeapYear(DateTime.Today.Year) && DateTime.Today.Month <= 2){
            return DateTime.Today.AddDays(-364);
        }
    }

Answer 1

Since you mention the "same week" I suppose you want to get the same day of the week in the same week number?

If so, you can do the following:

// In the System.DayOfWeek enum Sunday = 0, while Monday = 1
// This converts DateTime.DayOfWeek to a range where Monday = 0 and Sunday = 6
static int DayOfWeek(DateTime dt)
{
    const int weekStart = (int)System.DayOfWeek.Monday;
    const int daysInAWeek = 7;
    return (daysInAWeek - (weekStart - (int)dt.DayOfWeek)) % daysInAWeek;   
}

var calendar = CultureInfo.CurrentCulture.Calendar;

var weekNum = calendar.GetWeekOfYear(DateTime.Today, CalendarWeekRule.FirstFourDayWeek, System.DayOfWeek.Monday);

var todayLastYear = DateTime.Today.AddYears(-1);
var lastYearWeekNum = calendar.GetWeekOfYear(todayLastYear, CalendarWeekRule.FirstFourDayWeek, System.DayOfWeek.Monday);

var sameWeekLastYear = todayLastYear.AddDays(7 * (weekNum - lastYearWeekNum));
var sameDaySameWeekLastYear = sameWeekLastYear.AddDays(DayOfWeek(DateTime.Today) - DayOfWeek(sameWeekLastYear));

As you might notice there's a little convertion method, since I normally work with Monday being the first day of the week. If you prefer a different day to be the first day of the week, simply replace System.DayOfWeek.Monday with which ever day you'd like.

See this fiddle for a test run.