Get the number of calendar weeks between 2 dates in C#

Question

For the purposes of this question, let's assume the user will be from the US and will use the standard Gregorian calendar. So, a calendar week starts on Sunday and ends on Saturday.

What I'm trying to do is determine the number of calendar weeks that exist between two dates. A perfect example of my problem exists in October 2010. Between 10/16 and 10/31 there are 4 calendar weeks.

---

1. October 10 - October 16
   
2. October 17 - October 23
   
3. October 24 - October 30
   
4. October 31 - November 6
   

---

I'd prefer to stay away from any hardcoded logic like:

if (Day == DayOfWeek.Saturday && LastDayOfMonth == 31) { ... }

Can anyone think of a logical way to do this?

UPDATE:
Thanks for all the great responses, after some consideration here is the solution I used:

//get the start and end dates of the current pay period
DateTime currentPeriodStart = SelectedPeriod.Model.PeriodStart;
DateTime currentPeriodEnd = SelectedPeriod.Model.PeriodEnd;

//get the first sunday & last saturday span that encapsulates the current pay period
DateTime firstSunday = DayExtensions.SundayBeforePeriodStart(currentPeriodStart);
DateTime lastSaturday = DayExtensions.SaturdayAfterPeriodEnd(currentPeriodEnd);

//get the number of calendar weeks in the span
int numberOfCalendarWeeks = DayExtensions.CalendarWeeks(firstSunday, lastSaturday);

And here are the methods from the helper class:

    /// <summary>
    /// Get the first Sunday before the pay period start date
    /// </summary>
    /// <param name="periodStartDate">Date of the pay period start date</param>
    /// <returns></returns>
    public static DateTime SundayBeforePeriodStart(DateTime periodStartDate)
    {
        DateTime firstDayOfWeekBeforeStartDate;

        int daysBetweenStartDateAndPreviousFirstDayOfWeek = (int)periodStartDate.DayOfWeek - (int)DayOfWeek.Sunday;

        if (daysBetweenStartDateAndPreviousFirstDayOfWeek >= 0)
        {
            firstDayOfWeekBeforeStartDate = periodStartDate.AddDays(-daysBetweenStartDateAndPreviousFirstDayOfWeek);
        }
        else
        {
            firstDayOfWeekBeforeStartDate = periodStartDate.AddDays(-(daysBetweenStartDateAndPreviousFirstDayOfWeek + 7));
        }

        return firstDayOfWeekBeforeStartDate;
    }

    /// <summary>
    /// Get the first Saturday after the period end date
    /// </summary>
    /// <param name="periodEndDate">Date of the pay period end date</param>
    /// <returns></returns>
    public static DateTime SaturdayAfterPeriodEnd(DateTime periodEndDate)
    {
        DateTime lastDayOfWeekAfterEndDate;

        int daysBetweenEndDateAndFollowingLastDayOfWeek = (int)DayOfWeek.Saturday - (int)periodEndDate.DayOfWeek;

        if (daysBetweenEndDateAndFollowingLastDayOfWeek >= 0)
        {
            lastDayOfWeekAfterEndDate = periodEndDate.AddDays(daysBetweenEndDateAndFollowingLastDayOfWeek);
        }
        else
        {
            lastDayOfWeekAfterEndDate = periodEndDate.AddDays(daysBetweenEndDateAndFollowingLastDayOfWeek + 7);
        }

        return lastDayOfWeekAfterEndDate;
    }

    /// <summary>
    /// Get the calendar weeks between 2 dates
    /// </summary>
    /// <param name="d1">First day of date span</param>
    /// <param name="d2">Last day of date span</param>
    /// <returns></returns>
    public static int CalendarWeeks(DateTime d1, DateTime d2)
    {
        return 1 + (int)((d2 - d1).TotalDays / 7);
    }

And if you're curious, this is what I end up doing with the dates:

//create an array of all the sundays in this span
DateTime[] _sundays = new DateTime[numberOfCalendarWeeks];

//put the first sunday in the period
_sundays[0] = firstSunday;

//step through each week and get each sunday until you reach the last saturday
for (int i = 1; i <= numberOfCalendarWeeks - 1; i++)
{
     DateTime d = new DateTime();
     d = firstSunday.AddDays(i * 7);
      _sundays[i] = d;
}

for (int i = 0; i <= _sundays.Length-1; i++)
{
      //bind my view model with each sunday.
}

Answer 1

Here's a general solution which I believe should work for any choice of week starting and ending days. You could simplify it for your case, but this code gives you the option of changing the week's start and end (e.g. to Monday to Sunday) if it becomes necessary. It's not uncommon in payroll applications for the definition of a calendar week to change.

        DateTime periodStart = new DateTime(2010, 10, 17);
        DateTime periodEnd = new DateTime(2010, 11, 14);

        const DayOfWeek FIRST_DAY_OF_WEEK = DayOfWeek.Monday;
        const DayOfWeek LAST_DAY_OF_WEEK = DayOfWeek.Sunday;
        const int DAYS_IN_WEEK = 7;

        DateTime firstDayOfWeekBeforeStartDate;
        int daysBetweenStartDateAndPreviousFirstDayOfWeek = (int)periodStart.DayOfWeek - (int)FIRST_DAY_OF_WEEK;
        if (daysBetweenStartDateAndPreviousFirstDayOfWeek >= 0)
        {
            firstDayOfWeekBeforeStartDate = periodStart.AddDays(-daysBetweenStartDateAndPreviousFirstDayOfWeek);
        }
        else
        {
            firstDayOfWeekBeforeStartDate = periodStart.AddDays(-(daysBetweenStartDateAndPreviousFirstDayOfWeek + DAYS_IN_WEEK));
        }

        DateTime lastDayOfWeekAfterEndDate;
        int daysBetweenEndDateAndFollowingLastDayOfWeek = (int)LAST_DAY_OF_WEEK - (int)periodEnd.DayOfWeek;
        if (daysBetweenEndDateAndFollowingLastDayOfWeek >= 0)
        {
            lastDayOfWeekAfterEndDate = periodEnd.AddDays(daysBetweenEndDateAndFollowingLastDayOfWeek);
        }
        else
        {
            lastDayOfWeekAfterEndDate = periodEnd.AddDays(daysBetweenEndDateAndFollowingLastDayOfWeek + DAYS_IN_WEEK);
        }

        int calendarWeeks = 1 + (int)((lastDayOfWeekAfterEndDate - firstDayOfWeekBeforeStartDate).TotalDays / DAYS_IN_WEEK);

Answer 2

The way I would tackle this is to get the beginning of the week for any given date. Using that, you would subtract the result of the start date from the result of the end date. Your day difference would then always be a multiple of 7, so divide and add 1 (0 days => 1 week, 7 days => 2 weeks, etc.). This would tell you how many calendar weeks were covered or represented by any two dates.

static int GetWeeksCovered(DateTime startDate, DateTime endDate)
{
    if (endDate < startDate)
        throw new ArgumentException("endDate cannot be less than startDate");

    return (GetBeginningOfWeek(endDate).Subtract(GetBeginningOfWeek(startDate)).Days / 7) + 1;
}

static DateTime GetBeginningOfWeek(DateTime date)
{
    return date.AddDays(-1 * (int)date.DayOfWeek).Date;
}

• 16-Oct-2010 and 16-Oct-2010 => 1 week covered (or represented).
• 16-Oct-2010 and 31-Oct-2010 => 4 weeks covered, as per the spec.