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How to find the EndDate from a set of given inputs

I am struggling with a datetime problem where I am given a set of inputs and I need to find the EndDate by using those inputs. I am trying my best to solve this, but since I am running out of time, I landed up here. So someone who already might have faced this problem or someone with a solution could let me know one.
Problem Explanation:
The Concept is a Weekly schedule of a live class streaming application. The teacher has scheduled a weekly class from a specific start date.
Let's say, the start date is from 18th April, 2021 and the teacher selects 3 days per week(Monday, Tuesday & Wednesday) each with different class duration(By duration I mean that, We have the class start time of that day and the length of the class in hours and minutes).
Start Date: 18th April, 2021
Total days per week: 3
Days: Monday, Tuesday, Wednesday
Total duration per day: 3 hrs and 30 minutes
Total duration per week: 10 hours 30 minutes (Aggregated = Mon + Tue + Wed)
Max duration that the user cannot exceed: 50 hrs
Ok! Now we shall repeatedly add the TDPD(3 hrs and 30 minutes) to the start date until we reach 50 hrs and find which date it has landed upon(the end date).
What I have tried so far?
int totalWeeks = 0;
int totalHoursPerWeek = 3;
int totalHoursAdded = 0;
int maxHours = 50;
for(int i = totalHoursPerWeek; i <= maxHours; i += totalHoursPerWeek)
{
totalHoursAdded += i;
totalWeeks++;
}
Once the loop ends, I have the total week value.
DateTime endDate;
if(totalHoursAdded == maxHours)
{
//My problem is solved, as there is no remaining time pending
endDate = currentDate.AddDays(totalWeeks * 7);
}
else
{
// I have some pending hours
int pendingHours = maxHours - totalHoursAdded;
//How do I proceed with this pendingHours? how to add this to the
//specific days per week and find the end date? I am stuck here...
}

I am confident this can be done with some carefully thought-out math with dates, however below may be termed the lazy way out. This approach only needs the starting date, a list of days of the week that the class meets, i.e., Monday, Tuesday etc.…, the duration of the class in hours and minutes, and finally the max total hours and minutes that are required.
From my understanding, given the above info, we want to know, given that start date and the days of the week the class meets…
on what Date will the LAST class be that fulfills the max total hours.
To simplify this, one thing that will come in handy is knowing...
“how many classes are needed to fulfill the MAX requirement”.
In other words, if we know how many classes are needed to fulfill the max requirement, then this should make things easier.
Computing the total number of classes needed to fulfil the max requirement, can be found by dividing the max requirement by the class duration. If the division produces a remainder, then this would mean that one (1) additional class would be needed to fulfil the max requirement.
Therefore, if we have both the class duration and max duration variables as Timespan objects, then, we could divide the TimeSpan objects via their respective Tick properties and return how many classes are needed to fulfil the max requirement. This method may look something like…
private int GetTotalNumberOfClassesNeeded(TimeSpan classDuration, TimeSpan totalDuration) {
double td = totalDuration.Ticks / (double)classDuration.Ticks;
int totalClasses = (int)Math.Truncate(td); // <- get the whole portion
if (Math.Floor(td) != td) {
totalClasses++; // <- there is a fractional part - 1 more class needed
}
return totalClasses;
}
Next, we need to compare the DayOfWeek for a date with the DayOfWeek for the class. Therefore, is what we can do is create a List<DayOfWeek> … a list of DayOfWeek objects that the class is in session. We will use this list to check and see if a particular date’s day of week is IN that list. Therefore, in this example, the days of the week the class meets are a simple comma delimited string. Given this string, the code would parse out the days and return the proper list of DayOfWeek objects to compare with. This method may look something like…
private List<DayOfWeek> GetDaysOfWeekForClasses(string daysOfWeek) {
List<DayOfWeek> classesDOW = new List<DayOfWeek>();
string[] splitArray = daysOfWeek.Split(',');
DayOfWeek dow;
for (int i = 0; i < splitArray.Length; i++) {
switch (splitArray[i].Trim()) {
case "Monday":
dow = DayOfWeek.Monday;
break;
case "Tuesday":
dow = DayOfWeek.Tuesday;
break;
case "Wednesday":
dow = DayOfWeek.Wednesday;
break;
case "Thursday":
dow = DayOfWeek.Thursday;
break;
case "Friday":
dow = DayOfWeek.Friday;
break;
case "Saturday":
dow = DayOfWeek.Saturday;
break;
default:
dow = DayOfWeek.Sunday;
break;
}
classesDOW.Add(dow);
}
return classesDOW;
}
That is pretty much all we need. The general idea is this… we start by setting an int variable curClassCount to zero (0). In addition, we will create a DateTime object tempDate that is initialized with the starting date. Lastly, we will create a list of DateTime objects scheduledClasses which will get filled with the dates of the classes. We will start a while loop with the condition to continue as long as curClassCount is less than the total number of classes needed.
In each iteration of the loop a check is made to see if the tempDate’s DayOfWeek is one of the class’s DayOfWeek. … if it is, then we add that date to the scheduledClasses list and increment curClassCount. Finally increment tempDate by one (1) day, then start the loop over. Eventually, the curClassCount will equal the number of classes needed. This code may look something like…
while (curClassCount < totalNumberOfClassesNeeded) {
if (ClassDaysOfWeek.Contains(tempDate.DayOfWeek)) {
scheduledClasses.Add(tempDate.Date);
curClassCount++;
}
tempDate = tempDate.AddDays(1);
}
Putting all this together by droping a DateTimePicker, four (4) TextBoxes, a Button and a multi-line TextBox onto a new Winforms .Net Form may look something like…
Using the code below should complete the example.
private void Form1_Load(object sender, EventArgs e) {
dateTimePicker1.Value = new DateTime(2021, 4, 18);
TextBoxTotDaysPerWeek.Text = "3";
textBoxClassDays.Text = "Monday, Tuesday, Wednesday";
textBoxClassDuration.Text = "00:03:00:00";
textBoxMaxDuation.Text = "02:02:00:00";
}
private void btnCalculate_Click(object sender, EventArgs e) {
DateTime StartDate = dateTimePicker1.Value;
TimeSpan.TryParse(textBoxClassDuration.Text.Trim(), out TimeSpan ClassDuration);
TimeSpan.TryParse(textBoxMaxDuation.Text.Trim(), out TimeSpan MaxDuration);
int totalNumberOfClassesNeeded = GetTotalNumberOfClassesNeeded(ClassDuration, MaxDuration);
List<DayOfWeek> ClassDaysOfWeek = GetDaysOfWeekForClasses(textBoxClassDays.Text.Trim());
List<DateTime> scheduledClasses = new List<DateTime>();
int curClassCount = 0;
DateTime tempDate = StartDate.Date;
while (curClassCount < totalNumberOfClassesNeeded) {
if (ClassDaysOfWeek.Contains(tempDate.DayOfWeek)) {
scheduledClasses.Add(tempDate.Date);
curClassCount++;
}
tempDate = tempDate.AddDays(1);
}
txtBoxResults.Text = "";
txtBoxResults.Text = "Start Date: " + StartDate.ToShortDateString() +Environment.NewLine;
for (int i = 0; i < scheduledClasses.Count; i++) {
txtBoxResults.Text += "Class # " + (i + 1) + " of " + totalNumberOfClassesNeeded +
" Date: " + scheduledClasses[i].ToShortDateString() + Environment.NewLine;
}
}
A note, on the max duration… since the TimeSpan only allows hours < 23, we need to break 50 hours into 2 days and 2 hours. I hope this makes sense.

These are two options (using For and using While loop), should solve the problem:
using System;
namespace SO.DtProblem
{
class Program
{
static void Main(string[] args)
{
ClaculationOption1(); //Using For loop
ClaculationOption2(); //Using While loop
}
private static void ClaculationOption1()
{
var totalWeeks = 0;
var totalHoursPerWeek = 3;
var durationPerDay = 3;
var maxHours = 50;
var courseStartDate = new DateTime(2021, 4, 12); //Which is a Monday day
totalWeeks = maxHours / totalHoursPerWeek;
var expectedEndDate = courseStartDate.AddDays(totalWeeks * 7);
var pendingHours = maxHours % totalHoursPerWeek;
for (var day = 1; day <= 6; day++)
{
if (pendingHours > 0)
{
expectedEndDate = expectedEndDate.AddDays(1);
if ((expectedEndDate.AddDays(1).DayOfWeek == DayOfWeek.Monday)
|| (expectedEndDate.AddDays(1).DayOfWeek == DayOfWeek.Tuesday)
|| (expectedEndDate.AddDays(1).DayOfWeek == DayOfWeek.Wednesday))
{
if (pendingHours - durationPerDay >= 0)
{
pendingHours = pendingHours - durationPerDay;
}
else
{
pendingHours = 0;
break;
}
}
}
}
Console.Clear();
Console.WriteLine("Option 1 Results");
Console.WriteLine($"Course Start Date : {courseStartDate}");
Console.WriteLine($"Course Start Date Day Name: {courseStartDate.DayOfWeek}");
Console.WriteLine($"Expected End Date : {expectedEndDate}");
Console.WriteLine($"Expected End Date Day Name: {expectedEndDate.DayOfWeek}");
Console.WriteLine("===========================================================");
}
private static void ClaculationOption2()
{
var totalWeeks = 0;
var totalHoursPerWeek = 3;
var durationPerDay = 3;
var maxHours = 50;
var courseStartDate = new DateTime(2021, 4, 12); //Which is a Monday day
totalWeeks = maxHours / totalHoursPerWeek;
var expectedEndDate = courseStartDate.AddDays(totalWeeks * 7);
var pendingHours = maxHours % totalHoursPerWeek;
while (pendingHours > 0)
{
expectedEndDate = expectedEndDate.AddDays(1);
if ((expectedEndDate.AddDays(1).DayOfWeek == DayOfWeek.Monday)
|| (expectedEndDate.AddDays(1).DayOfWeek == DayOfWeek.Tuesday)
|| (expectedEndDate.AddDays(1).DayOfWeek == DayOfWeek.Wednesday))
{
if (pendingHours - durationPerDay >= 0)
{
pendingHours = pendingHours - durationPerDay;
}
else
{
pendingHours = 0;
break;
}
}
}
Console.WriteLine("Option 2 Results");
Console.WriteLine($"Course Start Date : {courseStartDate}");
Console.WriteLine($"Course Start Date Day Name: {courseStartDate.DayOfWeek}");
Console.WriteLine($"Expected End Date : {expectedEndDate}");
Console.WriteLine($"Expected End Date Day Name: {expectedEndDate.DayOfWeek}");
Console.ReadKey();
}
}
}

Finally I got my own solution working on the idea based on #John's answer. This answer is specially for the variable hours and minutes.
private static void CalculateClassDates()
{
DateTime courseStartDateTime = DateTime.Today;
int courseDurationInHours = 60;
int courseDurationInMinutes = 0;
TimeSpan courseMaxDuration = new TimeSpan(courseDurationInHours, courseDurationInMinutes, 0);
List<CourseScheduleDates> courseScheduleDates = new List<CourseScheduleDates>();
List<DaysOfWeek> daysOfWeeks = new List<DaysOfWeek>()
{
new DaysOfWeek(){ DayOfWeek = DayOfWeek.Monday, StartTime = DateTime.Today.AddHours(10).TimeOfDay , TotalHours = 1, TotalMinutes = 15},
new DaysOfWeek(){ DayOfWeek = DayOfWeek.Tuesday, StartTime = DateTime.Today.AddHours(10).TimeOfDay , TotalHours = 1, TotalMinutes = 15},
new DaysOfWeek(){ DayOfWeek = DayOfWeek.Wednesday, StartTime = DateTime.Today.AddHours(10).TimeOfDay , TotalHours = 1, TotalMinutes = 30}
};
int daysToAdd = 0;
TimeSpan singleDuration = new TimeSpan(daysOfWeeks[0].TotalHours, daysOfWeeks[0].TotalMinutes, 0);
TimeSpan additionallyAddedTime = TimeSpan.Zero;
List<DayOfWeek> days = daysOfWeeks.Select(x => x.DayOfWeek).ToList();
while (singleDuration.Ticks <= courseMaxDuration.Ticks)
{
if (days.Contains(courseStartDateTime.AddDays(daysToAdd).DayOfWeek))
{
var dayOfWeek = daysOfWeeks.Where(x => x.DayOfWeek == courseStartDateTime.AddDays(daysToAdd).DayOfWeek).First();
courseScheduleDates.Add(new CourseScheduleDates()
{
ScheduleDate = courseStartDateTime.AddDays(daysToAdd),
StartTime = dayOfWeek.StartTime,
TotalHours = dayOfWeek.TotalHours,
TotalMinutes = dayOfWeek.TotalMinutes
});
additionallyAddedTime = new TimeSpan(dayOfWeek.TotalHours, dayOfWeek.TotalMinutes, 0);
singleDuration = singleDuration.Add(additionallyAddedTime);
}
daysToAdd++;
}
singleDuration = singleDuration.Subtract(additionallyAddedTime);
if (singleDuration.Ticks != courseMaxDuration.Ticks)
{
var timeSpanToAdd = new TimeSpan(courseMaxDuration.Ticks - singleDuration.Ticks);
bool shouldContinue = true;
while (shouldContinue)
{
var currentDate = courseStartDateTime.AddDays(daysToAdd);
if (days.Contains(currentDate.DayOfWeek))
{
var dayOfWeek = daysOfWeeks.Where(x => x.DayOfWeek == courseStartDateTime.DayOfWeek).First();
courseScheduleDates.Add(new CourseScheduleDates()
{
ScheduleDate = courseStartDateTime.AddDays(daysToAdd),
StartTime = dayOfWeek.StartTime,
TotalHours = timeSpanToAdd.Hours,
TotalMinutes = timeSpanToAdd.Minutes
});
shouldContinue = false;
}
}
}
Console.WriteLine("Course Start Date " + courseStartDateTime.ToString("dd MMM, yyyy"));
int classCount = 1;
foreach (var item in courseScheduleDates)
{
DateTime dateTime = new DateTime(item.StartTime.Ticks);
Console.WriteLine("Class " + classCount + " will commence on " + item.ScheduleDate.ToString("dd MMM, yyyy") +
" " + dateTime.ToString("hh:mm tt") + " and will last for " + item.TotalHours + " hrs " + item.TotalMinutes + " mins");
classCount++;
}
Console.ReadLine();
}
And the Models
public class DaysOfWeek
{
public DayOfWeek DayOfWeek { get; set; }
public TimeSpan StartTime { get; set; }
public int TotalHours { get; set; }
public int TotalMinutes { get; set; }
}
public class CourseScheduleDates
{
public DateTime ScheduleDate { get; set; }
public TimeSpan StartTime { get; set; }
public int TotalHours { get; set; }
public int TotalMinutes { get; set; }
}

Calculating no of weekends in between the two given date and time fields by considering the time difference and output the difference in minutes

I have two fields startdate and enddate. I need to calculate how many weekends in between two date and time fields and show the result in minutes.
For example start date is 01/11/2019 00:00:00 and end date as 03/11/2019 11:00:00. Below code is returning the difference in minutes correctly as 2100 minutes but when I keep the dates as02/11/2019 08:00 and 03/11/2019 00:00 I am getting the result as 1440 but my expected result is 960 minutes.
I understand that's because I am adding 1440 in code so how to correct this?
public double CountOfWeekEnds(DateTime startDate, DateTime endDate)
{
double weekEndCount = 0;
if (startDate > endDate)
{
DateTime temp = startDate;
startDate = endDate;
endDate = temp;
}
TimeSpan diff = endDate - startDate;
int days = diff.Days;
for (var i = 0; i <= days; i++)
{
var testDate = startDate.AddDays(i);
if (testDate.DayOfWeek == DayOfWeek.Saturday || testDate.DayOfWeek == DayOfWeek.Sunday)
{
if (testDate.Date < endDate.Date)
{
weekEndCount += 1440; // 24h * 60 min
}
else
{
var todayStart = new DateTime(testDate.Year, testDate.Month, testDate.Day, 0, 0, 0);
var difference = (endDate - todayStart).TotalMinutes;
weekEndCount += difference;
}
}
}
return weekEndCount;
}

OK, i simplified what i said a little down to:
DateTime start = new DateTime(2019,11,1,0,0,0);
DateTime end = new DateTime(2019, 11, 3, 11, 0, 0);
TimeSpan diff = end - start;
Console.WriteLine(diff.TotalDays);
int total = 0;
for (int i = 0; i<Math.Ceiling(diff.TotalDays); i++)
{
DateTime test = start.AddDays(i);
Console.WriteLine(test.DayOfWeek);
if (test.DayOfWeek == DayOfWeek.Saturday || test.DayOfWeek == DayOfWeek.Sunday)
{
if (test.Date==start.Date)
{
Console.WriteLine("start");
total += (23 - start.Hour) * 60 + (60 - start.Minute);
}
else if (test.Date==end.Date)
{
Console.WriteLine("end");
total += end.Hour * 60 + end.Minute;
}
else
{
total += 24 * 60;
}
}
Console.WriteLine(test + " total " + total);
}
Console.WriteLine("done");
Console.WriteLine(total);
which counts all saturdays and sundays and allows for start and ends to be partials
(and can someone send a keyboard with actual keys this membrain lark is hampering typings)

Trying to remain as much of the original code as possible, only three minor changes have to be made:
1. Use the actual dates to calculate diff:
TimeSpan diff = endDate.Date - startDate.Date; instead of TimeSpan diff = endDate - startDate;
This is because later in the upcoming for-loop you are trying to evaluate each date in order to say if is a saturday or sunday. Otherwise, you are evaluating if the date 24 (, 48, …) hours after your starting time stamp is a saturday or sunday.
2. Use testDate instead of todayStart in order to calculate difference
difference = (endDate - testDate).TotalMinutes;
instead of
var todayStart = new DateTime(testDate.Year, testDate.Month, testDate.Day, 0, 0, 0);
var difference = (endDate - todayStart).TotalMinutes;
This is because testDate does contain the hours and minutes to calculate the difference in minutes. Otherwise you are just ignoring the day time of the starting day. Note that this correction can lead to a negative difference value if the startDate day time is later than the endDate day time.
3. do not add a whole day if there is only one day to examine in total
That means that if startDate.Date == endDate.Date, you should just calculate the difference between the dates.
if (testDate.Date < endDate.Date && startDate.Date != endDate.Date)
This has to be done because of the code logic: a full day is added for every new day other than the final day and for the final day ~24hours are added or substracted to the final value depending on the day times of the startDate and endDate.
The complete corrected code:
public static double CountOfWeekEnds(DateTime startDate, DateTime endDate)
{
double weekEndCount = 0;
if (startDate > endDate)
{
DateTime temp = startDate;
startDate = endDate;
endDate = temp;
}
TimeSpan diff = endDate.Date - startDate.Date; //instead of endDate - startDate
int days = diff.Days;
for (var i = 0; i <= days; i++)
{
var testDate = startDate.AddDays(i);
//Console.WriteLine(testDate);
if (testDate.DayOfWeek == DayOfWeek.Saturday || testDate.DayOfWeek == DayOfWeek.Sunday) //only weekends count
{
if (testDate.Date < endDate.Date && startDate.Date != endDate.Date) { // added startDate.Date != endDate.Date
weekEndCount += 1440; // 24h * 60 min
//Console.WriteLine("************************add 1440 ");
}
else
{
double difference;
difference = (endDate - testDate).TotalMinutes; //instead of endDate - todayStart
//Console.WriteLine("************************add " + difference);
weekEndCount += difference;
}
}
}
//return days;
return weekEndCount;
}

You need to have a look at this condition:
if (testDate.Date < endDate.Date)
It means that "as long as the ticks of testDate is less than the ticks of endDate".
This condition will be true for all conditions that makes your variable "days" positive.
I think you need to extend this, condition e.g.
if ((endDate - todayStart).TotalMinutes > 1440 )
This way it will check whether it is AT LEAST 24 hours earlier. If it isn't it should go forth with your "else" condition and take the used fraction of the start day into consideration.

Here is a (somewhat) simple solution. Please note that the code could (and probably should) be refactored if it was to be production code. But I tried to optimize it for understandability, since it was your first post...
public static int CalculateWeekendMinutes(DateTime start, DateTime end)
{
int weekendMinutes = 0;
// First and last day will be handled seperately in the end
var firstFullDay = start.AddDays(1).Date;
var lastFullDay = end.AddDays(-1).Date;
TimeSpan limitedSpan = lastFullDay - firstFullDay;
int spanLengthDays = (int)limitedSpan.TotalDays;
var dateIterator = firstFullDay;
// Looping over the limited span allows us to analyse all the full days
while (dateIterator <= lastFullDay)
{
if (dateIterator.DayOfWeek == DayOfWeek.Saturday || dateIterator.DayOfWeek == DayOfWeek.Sunday)
{
weekendMinutes += (24 * 60);
}
dateIterator = dateIterator.AddDays(1);
}
// Finally we can calculate the partial days and add that to our total
weekendMinutes += CalculateMinutesOnFirstDay(start);
weekendMinutes += CalculateMinutesOnLastDay(end);
return weekendMinutes;
}
// Helps us calculate the minutes of the first day in the span
private static int CalculateMinutesOnFirstDay(DateTime date)
{
if (date.DayOfWeek == DayOfWeek.Saturday || date.DayOfWeek == DayOfWeek.Sunday)
{
// We want to know how many minutes there are UNTIL the next midnight
int minutes = (int)(date.Date.AddDays(1) - date).TotalMinutes;
return minutes;
}
else
{
return 0;
}
}
// Helps us calculate the minutes of the last day in the span
private static int CalculateMinutesOnLastDay(DateTime date)
{
if (date.DayOfWeek == DayOfWeek.Saturday || date.DayOfWeek == DayOfWeek.Sunday)
{
// We want to know how many minutes there are SINCE the last midnight
int minutes = (int)(date - date.Date).TotalMinutes;
return minutes;
}
else
{
return 0;
}
}

Count occurrence for each day of the week between two dates

I want to count occurrences, for each day of the week, between two given dates.
For example:
Between 20/07/2014 to 27/7/2014, an 8 day span, there were:
Sunday=2, monday=1, tuesday=1,...

Try this:
DateTime start = new DateTime(2014,07,20);
DateTime end = new DateTime(2014,07,27);
TimeSpan ts = end - start;
int limit = ts.Days;
var result = Enumerable.Range(0,limit+1)
.Select(x => start.AddDays(x))
.GroupBy(x => x.DayOfWeek)
.Select(x => new {day = x.Key, count = x.Count()});
We create a range of dates from start to end, inclusive of both dates, and then group by the day of week to get the days and corresponding counts.
Demo

You better convert the days first to DateTime instances:
DateTime d1 = new DateTime(2014,07,20);
DateTime d2 = new DateTime(2014,07,27);
Next you calculate the total days between the two dates:
int days = (int) Math.Floor((d2-d1).TotalDays)+1;
As well as the day of the week of the first date:
int dow = (int) d1.DayOfWeek;
Now we devide the number of days by seven and assign that number to all days: since this is the minimum occurences for each day:
int d7 = days/7;
int[] counts = new int[7];
for(int i = 0; i < 7; i++) {
counts[i] = d7;
}
The remainder of the days are distributed with the day of the week of d1 first:
int remainder = days-7*d7;
int dowi = dow;
while(remainder > 0) {
counts[dowi]++;
dowi = (dowi+1)%7;//next day of the week
remainder--;
}
Then we can return the arrray:
return counts;
Full method:
public static int[] countDelta (DateTime d1, DateTime d2) {
int days = (int) Math.Floor((d2-d1).TotalDays)+1;
int dow = (int) d1.DayOfWeek;
int d7 = days/7;
int[] counts = new int[7];
for(int i = 0; i < 7; i++) {
counts[i] = d7;
}
int remainder = days-7*d7;
int dowi = dow;
while(remainder > 0) {
counts[dowi]++;
dowi = (dowi+1)%7;//next day of the week
remainder--;
}
return counts;
}
The result of a csharp interactive session:
csharp> Foo.countDelta(new DateTime(2014,07,20),new DateTime(2014,07,27));
{ 2, 1, 1, 1, 1, 1, 1 }
The method runs in constant time (if the dates differ much, this will not have an impact on performance). The only constraint is that calendar must be modern: if somewhere in history, people skipped a few "days of the week", this could result in some problems.

You can try something like this. It may be not the best solution, but it's the first that came in mind:
var startDate = DateTime.Now;
var endDate = startDate.AddDays(15);
var rusultDictionary = new Dictionary<DayOfWeek, int>();
while (!startDate.Date.Equals(endDate.Date))
{
rusultDictionary[startDate.DayOfWeek] = rusultDictionary.ContainsKey(startDate.DayOfWeek) ? rusultDictionary[startDate.DayOfWeek] + 1 : 1;
startDate = startDate.AddDays(1);
}

the simple way is for loop each day in the period, and use switch case or if else to check the current date.DayOfWeek to count and save the week number, something like the following:
int mondays = 0;
switch(current.DayOfWeek){
case DayOfWeek.Monday:
mondays++;
break;
...
}

public static Dictionary<DayOfWeek, int> CountDayOfWeeks(DateTime #from, DateTime to)
{
var start = #from.Date;
var ret = Enum.GetValues(typeof(DayOfWeek)).Cast<DayOfWeek>().ToDictionary(x => x, x => 0);
while (start <= to)
{
ret[start.DayOfWeek]++;
start = start.AddDays(1);
}
return ret;
}

you can achieve it using Linq:
DateTime StartDate = DateTime.Now.AddDays(-14);
DateTime EndDate = DateTime.Now;
DayOfWeek day = DayOfWeek.Monday;
First get the all days between two dates:
List<DateTime> dates = Enumerable.Range(0, (int)((EndDate - StartDate).TotalDays) + 1)
.Select(n => StartDate.AddDays(n))
.ToList();
Now get Count on the base of day, currently it will get Count of Monday:
var MondayCount = dates.Count(x => x.DayOfWeek == day);
FIDDLE:
https://dotnetfiddle.net/ZopkFY

Compare two DATE and get Days in c#

Count Number of days by comapaing two date, When you want to compare two date like due Date and return date of a book in library then you can get no of days in this manner
int TotalDay;
DateTime due = OldDate;
int day = due.Day;
int nday = DateTime.Now.Day;
int mnt = due.Month;
int nmnt = DateTime.Now.Month;
int yr = due.Year;
int nyr = DateTime.Now.Year;
if (nyr <= yr)
{
if (nmnt <= mnt)
{
if (nday > day)
{
TotalDay = nday - day;
}
}
else
{
TotalDay = nday - day;
m = nmnt - mnt;
TotalDay = d + (m * 30);
}
}
else
{
TotalDay = nday - day;
m = nmnt - mnt;
TotalDay = d + (m * 30);
int y = nyr - yr;
TotalDay = d + (y * 365);
}

Use TimeSpan
TimeSpan ts = dateTime1 - dateTime2;
ts.TotalDays will give you the difference in number of days.
In your case due is the due date and DateTime.Now is the current date. You may use:
TimeSpan ts = DateTime.Now - due;
or use the TimeSpan.TotalDays property:
TimeSpan ts = DateTime.Now.Subtract(due);
double NumberOfDays = ts.TotalDays;

You could use TimeSpan as shown in other answers - but you need to be careful about the time of day. Are you actually trying to think of these as dates or as dates with times? What about time zones? Are you using "local" DateTime values for both?
While DateTime can handle all of this, it leaves things unclear. I'd personally use Noda Time (which shouldn't surprise anyone as it's my own library...)
LocalDate startDate = ...;
LocalDate endDate = ...;
long days = Period.Between(startDate, endDate, PeriodUnits.Days).Days;
Note that finding "today's date" requires knowing the appropriate time zone, too. (DateTime.Now and DateTime.Today assume the system time zone where your code is running, but that may or may not be what you want.)

You need TimeSpan
Timespan t = ReturnDateTime - DateTime.Now;
Then use
t.Days
for calculating how many days are left.

DateTime Compare in c#

I want to compare two dateTime.
Ex:
date1 = 13/01/2004 12:20:00
date2 = 13/01/2004 12:35:00
result = Compare(date2-date1);
O/P : 15 Minutes

To compare, you can simply use the < operator: date1 < date2.
If you want to compare with a given resolution, try date1.TotalMinutes == date2.TotalMinutes (this compared for the same minute).
If you want to know if the difference is within a certain time span, use this:
System.TimeSpan dt = date2.Subtract(date1);
if (dt.TotalMinutes < 15) //...

Try this:
TimeSpan diff = date2.Subtract(date1);

How about
if (date1 < date2)
{
// date1 is before date2
}

You can use
double minutes = d2.Subtract(d1).TotalMinutes;
To get the total difference in minutes.

How about:
Timespan ts = date2 - date1;
Console.WriteLine("Value of Minutes = ", ts.Minutes);

I don't fully understand what you're asking.
If you want your pseudo-code expressing in C# here you go...
//date1 = 13/01/2004 12:20:00
DateTime dateTime1 = new DateTime(2004, 01, 13, 12, 20, 0);
//date2 = 13/01/2004 12:35:00
DateTime dateTime2 = new DateTime(2004, 01, 13, 12, 35, 0);
//get the time difference - result = Compare(date2-date1);
TimeSpan result = dateTime2 - dateTime1;
//output is 15
Console.WriteLine(result.TotalMinutes);

DateTime date1 = DateTime.Now;
DateTime date2 = DateTime.Now;
var x = date1.CompareTo(date2);
EDIT: I see now that you wanted to get the time difference between the two dates. For that you use the TimeSpan class.

Now this is the best bet.
using System;
public class Example
{
public static void Main()
{
DateTime date1 = new DateTime(2009, 8, 1, 0, 0, 0);
DateTime date2 = new DateTime(2009, 8, 1, 12, 0, 0);
int result = DateTime.Compare(date1, date2);
string relationship;
if (result < 0)
relationship = "is earlier than";
else if (result == 0)
relationship = "is the same time as";
else
relationship = "is later than";
Console.WriteLine("{0} {1} {2}", date1, relationship, date2);
}
}
// The example displays the following output:
// 8/1/2009 12:00:00 AM is earlier than 8/1/2009 12:00:00 PM

In the words of Larry Wall there is more than one way to do this. If you are looking for the -1, 0, +1 result of a compare within a certain time interval try one of these variants;
internal static int XDateCompare(DateTime date, DateTime other, int ticks)
{
var diff = date.Ticks - other.Ticks;
var result = Math.Abs(diff) <= ticks ? 0
: diff <= 0 ? -1
: 1;
Console.WriteLine("{0}\t{1}\t{2}\ts={3} milSec={4}", diff, other, result, ticks, date.Subtract(other).Duration().TotalMilliseconds);
return result;
}
internal static int XDateCompare(DateTime date, DateTime other, double milliseconds)
{
double diff =
date.Subtract(other)
.TotalMilliseconds;
var result = Math.Abs(diff) <= milliseconds ? 0
: diff <= 0 ? -1
: 1;
Console.WriteLine("result {0} diff {1} vs ms {2}", result, diff, milliseconds);
return result;
}