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  2. Is .NET giving me the wrong week number for Dec. 29th 2008?

Is .NET giving me the wrong week number for Dec. 29th 2008?

由 懵懂的女人 提交于 2019-11-30 08:32:52

This article looks deeper into the issue and possible workarounds. The hub of the matter is that the .NET calendar implementation does not seem to faithfully implement the ISO standard

@Conrad is correct. The .NET implementation of DateTime and the GregorianCalendar don't implement/follow the full ISO 8601 spec. That being said, they spec is extremely detailed and non-trivial to implement fully, at least for the parsing side of things.

Some more information is available at the following sites:

Simply put:

A week is identified by its number in a given year and begins with a Monday. The first week of a year is the one which includes the first Thursday, or equivalently the one which includes January 4.

Here is part of the code I use for properly handling ISO 8601 dates:

    #region FirstWeekOfYear
    /// <summary>
    /// Gets the first week of the year.
    /// </summary>
    /// <param name="year">The year to retrieve the first week of.</param>
    /// <returns>A <see cref="DateTime"/>representing the start of the first
    /// week of the year.</returns>
    /// <remarks>
    /// Week 01 of a year is per definition the first week that has the Thursday 
    /// in this year, which is equivalent to the week that contains the fourth
    /// day of January. In other words, the first week of a new year is the week
    /// that has the majority of its days in the new year. Week 01 might also 
    /// contain days from the previous year and the week before week 01 of a year
    /// is the last week (52 or 53) of the previous year even if it contains days 
    /// from the new year.
    /// A week starts with Monday (day 1) and ends with Sunday (day 7). 
    /// </remarks>
    private static DateTime FirstWeekOfYear(int year)
    {
        int dayNumber;

        // Get the date that represents the fourth day of January for the given year.
        DateTime date = new DateTime(year, 1, 4, 0, 0, 0, DateTimeKind.Utc);

        // A week starts with Monday (day 1) and ends with Sunday (day 7).
        // Since DayOfWeek.Sunday = 0, translate it to 7. All of the other values
        // are correct since DayOfWeek.Monday = 1.
        if (date.DayOfWeek == DayOfWeek.Sunday)
        {
            dayNumber = 7;
        }
        else
        {
            dayNumber = (int)date.DayOfWeek;
        }

        // Since the week starts with Monday, figure out what day that 
        // Monday falls on.
        return date.AddDays(1 - dayNumber);
    }

    #endregion

    #region GetIsoDate
    /// <summary>
    /// Gets the ISO date for the specified <see cref="DateTime"/>.
    /// </summary>
    /// <param name="date">The <see cref="DateTime"/> for which the ISO date
    /// should be calculated.</param>
    /// <returns>An <see cref="Int32"/> representing the ISO date.</returns>
    private static int GetIsoDate(DateTime date)
    {
        DateTime firstWeek;
        int year = date.Year;

        // If we are near the end of the year, then we need to calculate
        // what next year's first week should be.
        if (date >= new DateTime(year, 12, 29))
        {
            if (date == DateTime.MaxValue)
            {
                firstWeek = FirstWeekOfYear(year);
            }
            else
            {
                firstWeek = FirstWeekOfYear(year + 1);
            }

            // If the current date is less than next years first week, then
            // we are still in the last month of the current year; otherwise
            // change to next year.
            if (date < firstWeek)
            {
                firstWeek = FirstWeekOfYear(year);
            }
            else
            {
                year++;
            }
        }
        else
        {
            // We aren't near the end of the year, so make sure
            // we're not near the beginning.
            firstWeek = FirstWeekOfYear(year);

            // If the current date is less than the current years
            // first week, then we are in the last month of the
            // previous year.
            if (date < firstWeek)
            {
                if (date == DateTime.MinValue)
                {
                    firstWeek = FirstWeekOfYear(year);
                }
                else
                {
                    firstWeek = FirstWeekOfYear(--year);
                }
            }
        }

        // return the ISO date as a numeric value, so it makes it
        // easier to get the year and the week.
        return (year * 100) + ((date - firstWeek).Days / 7 + 1);
    }

    #endregion

    #region Week
    /// <summary>
    /// Gets the week component of the date represented by this instance.
    /// </summary>
    /// <value>The week, between 1 and 53.</value>
    public int Week
    {
        get
        {
            return this.isoDate % 100;
        }
    }
    #endregion

    #region Year
    /// <summary>
    /// Gets the year component of the date represented by this instance.
    /// </summary>
    /// <value>The year, between 1 and 9999.</value>
    public int Year
    {
        get
        {
            return this.isoDate / 100;
        }
    }
    #endregion

Week numbers differ from country to country and should depend on your locale/regional settings if I'm not entirely wrong.

Edit: Wikipedia supports my vague recollection that these numbers differ based on country: http://en.wikipedia.org/wiki/Week_number#Week_number

I would expect a respectable framework to obey the COUNTRY selected in your local runtime.

In my experience, the demonstrated behavior is the typical behavior, referring to the partial final week as week 53. This may be because all significant exposure I've had to week numbers has been related to accounting the end of the calendar year for reporting purposes, and the IRS (or tax agency of your choice) considers the calendar year to end on 31 December, not the last full week of the year.

I know this is an old post, but any way noda time seems to get the correct result..

As a work around, you could say the week number is WeekNumber mod 52. I believe this would work for the cases you describe.

As a workaround, why not use FirstFourDayWeek but add:

  if ( weekNumber > 52 )
    weekNumber = 1;
标签
c#
.net
calendar
gregorian-calendar
week-number
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