Count days between two dates with Java 8 while ignoring certain days of week

If we talk about a Java 8 API, why not use the Java 8 features consequently…

static long daysBetween(LocalDate start, LocalDate end, List<DayOfWeek> ignore) {
    return Stream.iterate(start, d->d.plusDays(1))
                 .limit(start.until(end, ChronoUnit.DAYS))
                 .filter(d->!ignore.contains(d.getDayOfWeek()))
                 .count();
}

Starting with Java 9, we can use the even simpler

static long daysBetween(LocalDate start, LocalDate end, List<DayOfWeek> ignore) {
    return start.datesUntil(end)
        .filter(d->!ignore.contains(d.getDayOfWeek()))
        .count();
}

Though, it might be worth using a Set with a better-than-linear lookup rather than the List:

static long daysBetween(LocalDate start, LocalDate end, List<DayOfWeek> ignore) {
    if(ignore.isEmpty()) return start.until(end, ChronoUnit.DAYS);
    EnumSet<DayOfWeek> set = EnumSet.copyOf(ignore);
    return start.datesUntil(end)
        .filter(d->!ignore.contains(d.getDayOfWeek()))
        .count();
}

You may consider changing the parameter to Set<DayOfWeek>, as it is not only more efficient but better suited to the actual use cases. Instead of Arrays.asList(DayOfWeek.SUNDAY, DayOfWeek.WEDNESDAY, DayOfWeek.FRIDAY), you can pass EnumSet.of(DayOfWeek.SUNDAY, DayOfWeek.WEDNESDAY, DayOfWeek.FRIDAY), but you can also use constructs like EnumSet.range(DayOfWeek.MONDAY, DayOfWeek.FRIDAY), to denote the typical working days.

You can avoid iterating over all days, but it requires special care about corner cases and hence, thorough testing. And will only pay off for really large ranges. For completeness, this is the optimized variant:

static long daysBetween(LocalDate start, LocalDate end, Set<DayOfWeek> ignore) {
    long d1 = start.toEpochDay(), d2 = end.toEpochDay();
    if(d1 > d2) throw new IllegalArgumentException();
    if(ignore.isEmpty()) return d2 - d1;
    int incompleteWeek = 0;
    DayOfWeek startDoW = start.getDayOfWeek(), endDoW = end.getDayOfWeek();
    if(startDoW != endDoW) {
        for(int v1 = startDoW.getValue(), v2 = endDoW.getValue();
            v1 != v2 && d1 < d2; v1 = v1%7+1, d1++) {
                if(!ignore.contains(DayOfWeek.of(v1))) incompleteWeek++;
        }
    }
    return incompleteWeek + (d2 - d1) * (7 - ignore.size()) / 7;
}

Here, the performance of the ignore set’s lookup doesn’t matter, as we only look up at most six values, however, enforcing a Set, i.e. no duplicates, allows us to use the set’s size to calculate the number of days contained in complete weeks of the range. Complete weeks have the same day of week for the start and (exclusive) end date. So the code only needs to iterate the days, until the start and end day of week match.

You you are using wrong Excel formula. See the section "Using SUM and INT function to count the number of workdays" of the site that you have provided. It is stating the formula as:

=SUM(INT((WEEKDAY(A2-{2,3,4,5,6})+B2-A2)/7))

In Excel, Sunday is 1 and Saturday is 7. The numbers inside the curly braces indicates the day-of-weeks to be included. So for your case the formula will be:

=SUM(INT((WEEKDAY(A2-{2,3,5,7})+B2-A2)/7))

Please see the attached screenshot:

It is returning 2247 as the following code returns:

import java.time.DayOfWeek;
import java.time.LocalDate;
import java.time.Month;
import java.time.Year;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoUnit;


public class SO25798876 {

    public static void main(String[] args) {
        String strStartDate = "09/07/2014";
        String strEndDate = "06/13/2025";
        String pattern = "MM/dd/yyyy";
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern(pattern);
        LocalDate startDate = LocalDate.parse(strStartDate, formatter);
        LocalDate endDate = LocalDate.parse(strEndDate, formatter);

        int count = 0;

        while(startDate.isBefore(endDate) || startDate.isEqual(endDate)) {  // you may want to not to use the isEqual method        
            DayOfWeek dayOfWeek = startDate.getDayOfWeek();         

            if(!(dayOfWeek == DayOfWeek.SUNDAY || dayOfWeek == DayOfWeek.WEDNESDAY || dayOfWeek == DayOfWeek.FRIDAY)) {
                count++;
            }           

            startDate = startDate.plusDays(1);
        }

        System.out.println(count);  
    }
}

You also have mentioned your doubt that the java.time may be not considering leap year, which is wrong, if you add the following piece of code

long year = startDate.getYear();

if(Year.isLeap(year)) {
    Month month = startDate.getMonth();

    if(month == Month.FEBRUARY && startDate.getDayOfMonth() == 29) {
        System.out.println("Calculated 29th Feb for the year: " + year);
    }
}

You will see that it is printing:

Calculated 29th Feb for the year: 2016
Calculated 29th Feb for the year: 2020
Calculated 29th Feb for the year: 2024

Lastly the count will be 2247 which matches the Excel result.

Happy coding.

-Tapas