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


1.

const int firstDayOfWeek = 0;  // 0 = Sunday
int wasteDaysStart = (7 + Convert.ToInt32(first.DayOfWeek) - firstDayOfWeek) % 7;
int diff = (int)(((last - first).TotalDays + wasteDaysStart + 6) / 7);
diff = diff + 1;

====================================================
2.

DateTime periodStart = first;
DateTime periodEnd = last;

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);

return calendarWeeks;

====================================================

Ref  : http://stackoverflow.com/questions/3065897/get-the-number-of-calendar-weeks-between-2-dates-in-c-sharp
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