# [ACCEPTED]-Count number of Mondays in a given date range-datediff

Try this:

```
static int CountDays(DayOfWeek day, DateTime start, DateTime end)
{
TimeSpan ts = end - start; // Total duration
int count = (int)Math.Floor(ts.TotalDays / 7); // Number of whole weeks
int remainder = (int)(ts.TotalDays % 7); // Number of remaining days
int sinceLastDay = (int)(end.DayOfWeek - day); // Number of days since last [day]
if (sinceLastDay < 0) sinceLastDay += 7; // Adjust for negative days since last [day]
// If the days in excess of an even week are greater than or equal to the number days since the last [day], then count this one, too.
if (remainder >= sinceLastDay) count++;
return count;
}
```

0

Since you're using C#, if you're using C#3.0, you 5 can use LINQ.

Assuming you have an Array/List/IQueryable 4 etc that contains your dates as DateTime 3 types:

```
DateTime[] dates = { new DateTime(2008,10,6), new DateTime(2008,10,7)}; //etc....
var mondays = dates.Where(d => d.DayOfWeek == DayOfWeek.Monday); // = {10/6/2008}
```

Added:

Not sure if you meant grouping 2 them and counting them, but here's how to 1 do that in LINQ as well:

```
var datesgrouped = from d in dates
group d by d.DayOfWeek into grouped
select new { WeekDay = grouped.Key, Days = grouped };
foreach (var g in datesgrouped)
{
Console.Write (String.Format("{0} : {1}", g.WeekDay,g.Days.Count());
}
```

It's fun to look at different algorithms 9 for calculating day of week, and @Gabe Hollombe's 8 pointing to WP on the subject was a great 7 idea (and I remember implementing Zeller's Congruence in COBOL 6 about twenty years ago), but it was rather 5 along the line of handing someone a blueprint 4 of a clock when all they asked what time 3 it was.

In C#:

```
private int CountMondays(DateTime startDate, DateTime endDate)
{
int mondayCount = 0;
for (DateTime dt = startDate; dt < endDate; dt = dt.AddDays(1.0))
{
if (dt.DayOfWeek == DayOfWeek.Monday)
{
mondayCount++;
}
}
return mondayCount;
}
```

This of course does not evaluate 2 the end date for "Mondayness", so 1 if this was desired, make the for loop evaluate

```
dt < endDate.AddDays(1.0)
```

Here's some pseudocode:

```
DifferenceInDays(Start, End) / 7 // Integer division discarding remainder
+ 1 if DayOfWeek(Start) <= DayImLookingFor
+ 1 if DayOfWeek(End) >= DayImLookingFor
- 1
```

Where `DifferenceInDays`

returns `End - Start`

in 8 days, and `DayOfWeek`

returns the day of the week as 7 an integer. It doesn't really matter what 6 mapping `DayOfWeek`

uses, as long as it is increasing 5 and matches up with `DayImLookingFor`

.

Note that this algorithm 4 assumes the date range is inclusive. If 3 `End`

should not be part of the range, you'll 2 have to adjust the algorithm slightly.

Translating 1 to C# is left as an exercise for the reader.

Any particular language and therefore date 7 format?

If dates are represented as a count 6 of days, then the difference between two 5 values plus one (day), and divide by 7, is 4 most of the answer. If both end dates are 3 the day in question, add one.

*Edited:* corrected 2 'modulo 7' to 'divide by 7' - thanks. And 1 that is integer division.

```
public List<DateTime> GetSelectedDaysInPeriod(DateTime startDate, DateTime endDate, List<DayOfWeek> daysToCheck)
{
var selectedDates = new List<DateTime>();
if (startDate >= endDate)
return selectedDates; //No days to return
if (daysToCheck == null || daysToCheck.Count == 0)
return selectedDates; //No days to select
try
{
//Get the total number of days between the two dates
var totalDays = (int)endDate.Subtract(startDate).TotalDays;
//So.. we're creating a list of all dates between the two dates:
var allDatesQry = from d in Enumerable.Range(1, totalDays)
select new DateTime(
startDate.AddDays(d).Year,
startDate.AddDays(d).Month,
startDate.AddDays(d).Day);
//And extracting those weekdays we explicitly wanted to return
var selectedDatesQry = from d in allDatesQry
where daysToCheck.Contains(d.DayOfWeek)
select d;
//Copying the IEnumerable to a List
selectedDates = selectedDatesQry.ToList();
}
catch (Exception ex)
{
//Log error
//...
//And re-throw
throw;
}
return selectedDates;
}
```

0

Add the smallest possible number to make 3 the first day a Monday. Subtract the smallest 2 possible number to make the last day a Monday. Calculate 1 the difference in days and divide by 7.

Convert the dates to Julian Day Number, then 3 do a little bit of math. Since Mondays are 2 zero mod 7, you could do the calculation 1 like this:

```
JD1=JulianDayOf(the_first_date)
JD2=JulianDayOf(the_second_date)
Round JD1 up to nearest multiple of 7
Round JD2 up to nearest multiple of 7
d = JD2-JD1
nMondays = (JD2-JD1+7)/7 # integer divide
```

I have had the same need today. I started 8 with the *cjm* function since I don't understand 7 the *JonB* function and since the *Cyberherbalist* function is 6 not linear.

I had have to correct

```
DifferenceInDays(Start, End) / 7 // Integer division discarding remainder
+ 1 if DayOfWeek(Start) <= DayImLookingFor
+ 1 if DayOfWeek(End) >= DayImLookingFor
- 1
```

to

```
DifferenceInDays(Start, End) / 7 // Integer division discarding remainder
+ 1 if DayImLookingFor is between Start.Day and End.Day
```

With the 5 between function that return true if, starting 4 from the start day, we meet first the dayImLookingFor 3 before the endDay.

I have done the between 2 function by computing the number of day 1 from startDay to the other two days:

```
private int CountDays(DateTime start, DateTime end, DayOfWeek selectedDay)
{
if (start.Date > end.Date)
{
return 0;
}
int totalDays = (int)end.Date.Subtract(start.Date).TotalDays;
DayOfWeek startDay = start.DayOfWeek;
DayOfWeek endDay = end.DayOfWeek;
///look if endDay appears before or after the selectedDay when we start from startDay.
int startToEnd = (int)endDay - (int)startDay;
if (startToEnd < 0)
{
startToEnd += 7;
}
int startToSelected = (int)selectedDay - (int)startDay;
if (startToSelected < 0)
{
startToSelected += 7;
}
bool isSelectedBetweenStartAndEnd = startToEnd >= startToSelected;
if (isSelectedBetweenStartAndEnd)
{
return totalDays / 7 + 1;
}
else
{
return totalDays / 7;
}
}
```

This will return a collection of integers 4 showing how many times each day of the week 3 occurs within a date range

```
int[] CountDays(DateTime firstDate, DateTime lastDate)
{
var totalDays = lastDate.Date.Subtract(firstDate.Date).TotalDays + 1;
var weeks = (int)Math.Floor(totalDays / 7);
var result = Enumerable.Repeat<int>(weeks, 7).ToArray();
if (totalDays % 7 != 0)
{
int firstDayOfWeek = (int)firstDate.DayOfWeek;
int lastDayOfWeek = (int)lastDate.DayOfWeek;
if (lastDayOfWeek < firstDayOfWeek)
lastDayOfWeek += 7;
for (int dayOfWeek = firstDayOfWeek; dayOfWeek <= lastDayOfWeek; dayOfWeek++)
result[dayOfWeek % 7]++;
}
return result;
}
```

Or a slight variation 2 which lets you do FirstDate.TotalDaysOfWeeks(SecondDate) and 1 returns a Dictionary

```
public static Dictionary<DayOfWeek, int> TotalDaysOfWeeks(this DateTime firstDate, DateTime lastDate)
{
var totalDays = lastDate.Date.Subtract(firstDate.Date).TotalDays + 1;
var weeks = (int)Math.Floor(totalDays / 7);
var resultArray = Enumerable.Repeat<int>(weeks, 7).ToArray();
if (totalDays % 7 != 0)
{
int firstDayOfWeek = (int)firstDate.DayOfWeek;
int lastDayOfWeek = (int)lastDate.DayOfWeek;
if (lastDayOfWeek < firstDayOfWeek)
lastDayOfWeek += 7;
for (int dayOfWeek = firstDayOfWeek; dayOfWeek <= lastDayOfWeek; dayOfWeek++)
resultArray[dayOfWeek % 7]++;
}
var result = new Dictionary<DayOfWeek, int>();
for (int dayOfWeek = 0; dayOfWeek < 7; dayOfWeek++)
result[(DayOfWeek)dayOfWeek] = resultArray[dayOfWeek];
return result;
}
```

A bit Modified Code is here works and Tested 1 by me

```
private int CountDays(DayOfWeek day, DateTime startDate, DateTime endDate)
{
int dayCount = 0;
for (DateTime dt = startDate; dt < endDate; dt = dt.AddDays(1.0))
{
if (dt.DayOfWeek == day)
{
dayCount++;
}
}
return dayCount;
}
```

Example:

```
int Days = CountDays(DayOfWeek.Friday, Convert.ToDateTime("2019-07-04"),
Convert.ToDateTime("2019-07-27")).ToString();
```

I had a similar problem for a report. I 7 needed the number of workdays between two 6 dates. I could have cycled through the dates 5 and counted but my discrete math training 4 wouldn't let me. Here is a function I wrote 3 in VBA to get the number of workdays between 2 two dates. I'm sure .net has a similar 1 WeekDay function.

```
1
2 ' WorkDays
3 ' returns the number of working days between two dates
4 Public Function WorkDays(ByVal dtBegin As Date, ByVal dtEnd As Date) As Long
5
6 Dim dtFirstSunday As Date
7 Dim dtLastSaturday As Date
8 Dim lngWorkDays As Long
9
10 ' get first sunday in range
11 dtFirstSunday = dtBegin + ((8 - Weekday(dtBegin)) Mod 7)
12
13 ' get last saturday in range
14 dtLastSaturday = dtEnd - (Weekday(dtEnd) Mod 7)
15
16 ' get work days between first sunday and last saturday
17 lngWorkDays = (((dtLastSaturday - dtFirstSunday) + 1) / 7) * 5
18
19 ' if first sunday is not begin date
20 If dtFirstSunday <> dtBegin Then
21
22 ' assume first sunday is after begin date
23 ' add workdays from begin date to first sunday
24 lngWorkDays = lngWorkDays + (7 - Weekday(dtBegin))
25
26 End If
27
28 ' if last saturday is not end date
29 If dtLastSaturday <> dtEnd Then
30
31 ' assume last saturday is before end date
32 ' add workdays from last saturday to end date
33 lngWorkDays = lngWorkDays + (Weekday(dtEnd) - 1)
34
35 End If
36
37 ' return working days
38 WorkDays = lngWorkDays
39
40 End Function
```

```
private System.Int32 CountDaysOfWeek(System.DayOfWeek dayOfWeek, System.DateTime date1, System.DateTime date2)
{
System.DateTime EndDate;
System.DateTime StartDate;
if (date1 > date2)
{
StartDate = date2;
EndDate = date1;
}
else
{
StartDate = date1;
EndDate = date2;
}
while (StartDate.DayOfWeek != dayOfWeek)
StartDate = StartDate.AddDays(1);
return EndDate.Subtract(StartDate).Days / 7 + 1;
}
```

0

Four years later, I thought I'd run a test:

```
[TestMethod]
public void ShouldFindFridaysInTimeSpan()
{
//reference: http://stackoverflow.com/questions/248273/count-number-of-mondays-in-a-given-date-range
var spanOfSixtyDays = new TimeSpan(60, 0, 0, 0);
var setOfDates = new List<DateTime>(spanOfSixtyDays.Days);
var now = DateTime.Now;
for(int i = 0; i < spanOfSixtyDays.Days; i++)
{
setOfDates.Add(now.AddDays(i));
}
Assert.IsTrue(setOfDates.Count == 60,
"The expected number of days is not here.");
var fridays = setOfDates.Where(i => i.DayOfWeek == DayOfWeek.Friday);
Assert.IsTrue(fridays.Count() > 0,
"The expected Friday days are not here.");
Assert.IsTrue(fridays.First() == setOfDates.First(i => i.DayOfWeek == DayOfWeek.Friday),
"The expected first Friday day is not here.");
Assert.IsTrue(fridays.Last() == setOfDates.Last(i => i.DayOfWeek == DayOfWeek.Friday),
"The expected last Friday day is not here.");
}
```

My 2 use of `TimeSpan`

is a bit of overkill---actually 1 I wanted to query `TimeSpan`

directly.

More Related questions

We use cookies to improve the performance of the site. By staying on our site, you agree to the terms of use of cookies.