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.

View a picture of October 2010

- October 10 - October 16
- October 17 - October 23
- October 24 - October 30
- 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.
}
```