How to test if a DateTime is between 2 days of week (DayOfWeek)

Question

In C#, given an arbitrary set of DayOfWeek end points (like, DayOfWeek.Friday and DayOfWeek.Sunday) how would one test if an arbitrary date falls between those two days, inclusive?

Example:

// result == true; Oct 23, 2010 is a Saturday
var result = InBetweenDaysInclusive(new DateTime(2010, 10, 23),
                                    DayOfWeek.Friday,
                                    DayOfWeek.Sunday);

// result == true; Oct 22, 2010 is a Friday
result = InBetweenDaysInclusive(new DateTime(2010, 10, 22),
                                DayOfWeek.Friday,
                                DayOfWeek.Sunday);

// result == true; Oct 24, 2010 is a Sunday
result = InBetweenDaysInclusive(new DateTime(2010, 10, 24),
                                DayOfWeek.Friday,
                                DayOfWeek.Sunday);

// result == false; Oct 25, 2010 is a Monday
result = InBetweenDaysInclusive(new DateTime(2010, 10, 25),
                                DayOfWeek.Friday,
                                DayOfWeek.Sunday);

Thanks!

Answer 1

Thomas Levesque's answer is good here. Remember that DayOfWeek.Sunday is 0 and not 6 inside this enum.

This is problematic for me since in Norway Monday is the first day of the week, and not Sunday.

In that case, you should consider checking if the enum equals DayOfWeek.Sunday, and if it is to add 7 to the value before you do the comparison to make sure that Sunday is considered in the correct way.

If you have Sunday as the first day of week where you live, it's not a problem anyway ;)

Answer 2

I think this should work. It should also work regardless of what you consider to be the first day of the week:

private bool InBetweenDaysInclusive(DateTime dateToCheck, DayOfWeek lowerLimit, DayOfWeek upperLimit)
{
    CultureInfo ci = CultureInfo.CurrentCulture;

    int diffDateToCheckFirstDayOfWeek = dateToCheck.DayOfWeek - ci.DateTimeFormat.FirstDayOfWeek;
    if (diffDateToCheckFirstDayOfWeek < 0)
        diffDateToCheckFirstDayOfWeek += 7;

    int diffLowerLimitFirstDayOfWeek = lowerLimit - ci.DateTimeFormat.FirstDayOfWeek;
    if (diffLowerLimitFirstDayOfWeek < 0)
        diffLowerLimitFirstDayOfWeek += 7;

    int diffUpperLimitFirstDayOfWeek = upperLimit - ci.DateTimeFormat.FirstDayOfWeek;
    if (diffUpperLimitFirstDayOfWeek < 0)
        diffUpperLimitFirstDayOfWeek += 7;

    if (diffUpperLimitFirstDayOfWeek < diffLowerLimitFirstDayOfWeek)
        throw new Exception("The lower-limit day must be earlier in the week than the upper-limit day");

    return diffDateToCheckFirstDayOfWeek >= diffLowerLimitFirstDayOfWeek && diffDateToCheckFirstDayOfWeek <= diffUpperLimitFirstDayOfWeek;
}

Edit:
In case you wonder, the if < 0 and += 7 is to get around that subtracting two days isn't culture aware. The code could probably be made a bit cleaner, but you get the point I think.

Answer 3

This function is going to need two separate branches depending on whether the difference between start and end date is negative or positive/zero.

I could be totally off-base, but I think this works for all cases:

// No state in method, so made it static
public static bool InBetweenDaysInclusive(DateTime date, DayOfWeek start, DayOfWeek end)
{
    DayOfWeek curDay = date.DayOfWeek;

    if (start <= end)
    {
        // Test one range: start to end
        return (start <= curDay && curDay <= end);
    }
    else
    {
        // Test two ranges: start to 6, 0 to end
        return (start <= curDay || curDay <= end);
    }
}

For reference, your test data returned the following when I ran it and added Console.WriteLine for each result:

True
True
True
False

Edit: My last explanation was too vague. Here's a fixed one.

The trick is that if end < start, then you have two valid ranges: start to upper bound and lower bound to end. This would result in (start <= curDay && curDay <= upperBound) || curDay <= end && lowerBound <= curDay)

However, since they are bounds, curDay is always <= upperBound and >= lowerBound, thus we omit that code.

Answer 4

@Brad's point that any day of the week falls between any two days of the week is valid. However, we are assuming the two days of the week in question are ordered.

That is, when we say, "is October 30, 2010 (a Saturday) between Friday and Sunday?", we are really asking, "is October 30, 2010 either a Friday, a Saturday, or a Sunday?".

This observation allows us to break down the problem into two components and solve the full problem easily:

1) Determine if a particular day of the week is one of a particular set of days of the week (this is trivial).

2) Determine the set of days of the week that take you from one day to another. That is, we want a function that returns "Friday, Saturday, Sunday" when given "Friday" and "Sunday", and that returns "Monday, Tuesday, Wednesday, Thursday, Friday" when given "Monday" and "Friday". This is the tricky part of the problem.

To solve the second problem, we basically walk from the first day to the second day, returning all days in-between. To do this correctly, we have to account for the fact that the second day may be less than the first day (in the representational sense of Sunday = 0 being less than Friday = 5). So, we perform the "walk" in an integer space, and add 7 to the second day if it is less than the first day. We convert to days-of-the-week space (which is the integers modulo 7) on the "way out".

Below is the code and a series of tests that solve this. The "GetDaysBetweenInclusive" method solves problem #2, and "IsDayOfWeekBetween" adds the solution to problem #1 and solves the OP's problem.

Enjoy.

using System;
using System.Collections.Generic;
using System.Linq;
using NUnit.Framework;

namespace DayOfWeekUtilities
{
    public static class DayOfWeekHelpers
    {
        /// <summary>
        /// returns all days of the week, inclusive, from day1 to day2
        /// </summary>
        public static IEnumerable<DayOfWeek> GetDaysBetweenInclusive(DayOfWeek day1,
                                                                     DayOfWeek day2)
        {
            var final = (int)day2;
            if(day2 < day1)
            {
                final += 7;
            }
            var curr = (int)day1;
            do
            {
                yield return (DayOfWeek) (curr%7);
                curr++;
            } while (curr <= final);
        }

        /// <summary>
        /// returns true if the provided date falls on a day of the 
        /// week between day1 and day2, inclusive
        /// </summary>
        public static bool IsDayOfWeekBetween(this DateTime date,
                                              DayOfWeek day1,
                                              DayOfWeek day2)
        {
            return GetDaysBetweenInclusive(day1, day2).Contains(date.DayOfWeek);
        }
    }

    [TestFixture]
    public class Tests
    {
        [Test]
        public void Test()
        {
            Assert.IsTrue(new DateTime(2010, 10, 22).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));
            Assert.IsTrue(new DateTime(2010, 10, 23).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));
            Assert.IsTrue(new DateTime(2010, 10, 24).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));
            Assert.IsFalse(new DateTime(2010, 10, 25).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));
            Assert.IsFalse(new DateTime(2010, 10, 26).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));
            Assert.IsFalse(new DateTime(2010, 10, 27).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));
            Assert.IsFalse(new DateTime(2010, 10, 28).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));
            Assert.IsTrue(new DateTime(2010, 10, 29).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Sunday));

            Assert.IsTrue(new DateTime(2010, 10, 22).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 23).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 24).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 25).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 26).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 27).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 28).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));
            Assert.IsTrue(new DateTime(2010, 10, 29).IsDayOfWeekBetween(DayOfWeek.Friday, DayOfWeek.Friday));

            Assert.IsTrue(new DateTime(2010, 10, 22).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 23).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));
            Assert.IsFalse(new DateTime(2010, 10, 24).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));
            Assert.IsTrue(new DateTime(2010, 10, 25).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));
            Assert.IsTrue(new DateTime(2010, 10, 26).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));
            Assert.IsTrue(new DateTime(2010, 10, 27).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));
            Assert.IsTrue(new DateTime(2010, 10, 28).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));
            Assert.IsTrue(new DateTime(2010, 10, 29).IsDayOfWeekBetween(DayOfWeek.Monday, DayOfWeek.Friday));

            Assert.IsTrue(new DateTime(2010, 10, 22).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
            Assert.IsTrue(new DateTime(2010, 10, 23).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
            Assert.IsTrue(new DateTime(2010, 10, 24).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
            Assert.IsTrue(new DateTime(2010, 10, 25).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
            Assert.IsTrue(new DateTime(2010, 10, 26).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
            Assert.IsFalse(new DateTime(2010, 10, 27).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
            Assert.IsTrue(new DateTime(2010, 10, 28).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
            Assert.IsTrue(new DateTime(2010, 10, 29).IsDayOfWeekBetween(DayOfWeek.Thursday, DayOfWeek.Tuesday));
        }
    }
}