Getting the first and last day of a month, using a given DateTime object

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I want to get the first day and last day of the month where a given date lies in. The date comes from a value in a UI field.

If I'm using a time picker I could say

var maxDay = dtpAttendance.MaxDate.Day;

But I'm trying to get it from a DateTime object. So if I have this...

DateTime dt = DateTime.today;

How to get first day and last day of the month from dt?


DateTime structure stores only one value, not range of values. MinValue and MaxValue are static fields, which hold range of possible values for instances of DateTime structure. These fields are static and do not relate to particular instance of DateTime. They relate to DateTime type itself.

Suggested reading: static (C# Reference)

UPDATE: Getting month range:

DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);

Finding First and Last Day in Month - Power Tips, Year $month = $date.Month # create a new DateTime object set to the first day of a given month and year $startOfMonth = Get-Date -Year $year -Month $month  Recommended. Did you ever need the first and last day in a given month? Here is a simple approach: # specify the date you want to examine # default is today $date = Get-Date $year = $date.Year $month = $date.Month # create a new DateTime object set to the first day of a given month and year $startOfMonth = Get-Date -Year $year -Month $month -Day 1 -Hour 0 -Minute 0 -Second 0 -Millisecond 0 # add a month and subtract the smallest possible time unit $endOfMonth = ($startOfMonth).AddMonths(1).


This is more a long comment on @Sergey and @Steffen's answers. Having written similar code myself in the past I decided to check what was most performant while remembering that clarity is important too.

Result

Here is an example test run result for 10 million iterations:

2257 ms for FirstDayOfMonth_AddMethod()
2406 ms for FirstDayOfMonth_NewMethod()
6342 ms for LastDayOfMonth_AddMethod()
4037 ms for LastDayOfMonth_AddMethodWithDaysInMonth()
4160 ms for LastDayOfMonth_NewMethod()
4212 ms for LastDayOfMonth_NewMethodWithReuseOfExtMethod()
2491 ms for LastDayOfMonth_SpecialCase()
Code

I used LINQPad 4 (in C# Program mode) to run the tests with compiler optimization turned on. Here is the tested code factored as Extension methods for clarity and convenience:

public static class DateTimeDayOfMonthExtensions
{
    public static DateTime FirstDayOfMonth_AddMethod(this DateTime value)
    {
        return value.Date.AddDays(1 - value.Day);
    }

    public static DateTime FirstDayOfMonth_NewMethod(this DateTime value)
    {
        return new DateTime(value.Year, value.Month, 1);
    }

    public static DateTime LastDayOfMonth_AddMethod(this DateTime value)
    {
        return value.FirstDayOfMonth_AddMethod().AddMonths(1).AddDays(-1);
    }

    public static DateTime LastDayOfMonth_AddMethodWithDaysInMonth(this DateTime value)
    {
        return value.Date.AddDays(DateTime.DaysInMonth(value.Year, value.Month) - value.Day);
    }

    public static DateTime LastDayOfMonth_SpecialCase(this DateTime value)
    {
        return value.AddDays(DateTime.DaysInMonth(value.Year, value.Month) - 1);
    }

    public static int DaysInMonth(this DateTime value)
    {
        return DateTime.DaysInMonth(value.Year, value.Month);
    }

    public static DateTime LastDayOfMonth_NewMethod(this DateTime value)
    {
        return new DateTime(value.Year, value.Month, DateTime.DaysInMonth(value.Year, value.Month));
    }

    public static DateTime LastDayOfMonth_NewMethodWithReuseOfExtMethod(this DateTime value)
    {
        return new DateTime(value.Year, value.Month, value.DaysInMonth());
    }
}

void Main()
{
    Random rnd = new Random();
    DateTime[] sampleData = new DateTime[10000000];

    for(int i = 0; i < sampleData.Length; i++) {
        sampleData[i] = new DateTime(1970, 1, 1).AddDays(rnd.Next(0, 365 * 50));
    }

    GC.Collect();
    System.Diagnostics.Stopwatch sw = System.Diagnostics.Stopwatch.StartNew();
    for(int i = 0; i < sampleData.Length; i++) {
        DateTime test = sampleData[i].FirstDayOfMonth_AddMethod();
    }
    string.Format("{0} ms for FirstDayOfMonth_AddMethod()", sw.ElapsedMilliseconds).Dump();

    GC.Collect();
    sw.Restart();
    for(int i = 0; i < sampleData.Length; i++) {
        DateTime test = sampleData[i].FirstDayOfMonth_NewMethod();
    }
    string.Format("{0} ms for FirstDayOfMonth_NewMethod()", sw.ElapsedMilliseconds).Dump();

    GC.Collect();
    sw.Restart();
    for(int i = 0; i < sampleData.Length; i++) {
        DateTime test = sampleData[i].LastDayOfMonth_AddMethod();
    }
    string.Format("{0} ms for LastDayOfMonth_AddMethod()", sw.ElapsedMilliseconds).Dump();

    GC.Collect();
    sw.Restart();
    for(int i = 0; i < sampleData.Length; i++) {
        DateTime test = sampleData[i].LastDayOfMonth_AddMethodWithDaysInMonth();
    }
    string.Format("{0} ms for LastDayOfMonth_AddMethodWithDaysInMonth()", sw.ElapsedMilliseconds).Dump();

    GC.Collect();
    sw.Restart();
    for(int i = 0; i < sampleData.Length; i++) {
        DateTime test = sampleData[i].LastDayOfMonth_NewMethod();
    }
    string.Format("{0} ms for LastDayOfMonth_NewMethod()", sw.ElapsedMilliseconds).Dump();

    GC.Collect();
    sw.Restart();
    for(int i = 0; i < sampleData.Length; i++) {
        DateTime test = sampleData[i].LastDayOfMonth_NewMethodWithReuseOfExtMethod();
    }
    string.Format("{0} ms for LastDayOfMonth_NewMethodWithReuseOfExtMethod()", sw.ElapsedMilliseconds).Dump();

    for(int i = 0; i < sampleData.Length; i++) {
        sampleData[i] = sampleData[i].FirstDayOfMonth_AddMethod();
    }

    GC.Collect();
    sw.Restart();
    for(int i = 0; i < sampleData.Length; i++) {
        DateTime test = sampleData[i].LastDayOfMonth_SpecialCase();
    }
    string.Format("{0} ms for LastDayOfMonth_SpecialCase()", sw.ElapsedMilliseconds).Dump();

}
Analysis

I was surprised by some of these results.

Although there is not much in it the FirstDayOfMonth_AddMethod was slightly faster than FirstDayOfMonth_NewMethod in most runs of the test. However, I think the latter has a slightly clearer intent and so I have a preference for that.

LastDayOfMonth_AddMethod was a clear loser against LastDayOfMonth_AddMethodWithDaysInMonth, LastDayOfMonth_NewMethod and LastDayOfMonth_NewMethodWithReuseOfExtMethod. Between the fastest three there is nothing much in it and so it comes down to your personal preference. I choose the clarity of LastDayOfMonth_NewMethodWithReuseOfExtMethod with its reuse of another useful extension method. IMHO its intent is clearer and I am willing to accept the small performance cost.

LastDayOfMonth_SpecialCase assumes you are providing the first of the month in the special case where you may have already calculated that date and it uses the add method with DateTime.DaysInMonth to get the result. This is faster than the other versions, as you would expect, but unless you are in a desperate need for speed I don't see the point of having this special case in your arsenal.

Conclusion

Here is an extension method class with my choices and in general agreement with @Steffen I believe:

public static class DateTimeDayOfMonthExtensions
{
    public static DateTime FirstDayOfMonth(this DateTime value)
    {
        return new DateTime(value.Year, value.Month, 1);
    }

    public static int DaysInMonth(this DateTime value)
    {
        return DateTime.DaysInMonth(value.Year, value.Month);
    }

    public static DateTime LastDayOfMonth(this DateTime value)
    {
        return new DateTime(value.Year, value.Month, value.DaysInMonth());
    }
}

If you have got this far, thank you for time! Its been fun :¬). Please comment if you have any other suggestions for these algorithms.

c#, AddMonths() method. Then we create a new DateTime variable by using the month added DateTime object's year, month and first day (day number 1) such as​  from datetime import datetime given_date = datetime.today().date() first_day_of_month = given_date.replace(day=1) print("\nFirst day of month: ", first_day_of_month, " ") Method 2: In this, the timedetla of the datetime module is used to find the days from the first day to the given date and then subtract it from the given date to get the first day.


Getting month range with .Net API (just another way):

DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = new DateTime(date.Year, date.Month, DateTime.DaysInMonth(date.Year, date.Month));

Relative Formats - Manual, 'last' space dayname space 'of', Calculates the last week day of the current month​. (Example: "first wednesday of july 23rd, 2008" means "2008-07-02" because the specific phrase Get the first day of a month, in a typical calendar month, typically the Monday in the first $date = new DateTime('first day of this month'); An alternative version with no requirements outside of Python's standard library: import datetime import calendar def get_month_day_range(date): """ For a date 'date' returns the start and end date for the month of 'date'.


"Last day of month" is actually "First day of *next* month, minus 1". So here's what I use, no need for "DaysInMonth" method:

public static DateTime FirstDayOfMonth(this DateTime value)
{
    return new DateTime(value.Year, value.Month, 1);
}

public static DateTime LastDayOfMonth(this DateTime value)
{
    return value.FirstDayOfMonth()
        .AddMonths(1)
        .AddMinutes(-1);
}

NOTE: The reason I use AddMinutes(-1), not AddDays(-1) here is because usually you need these date functions for reporting for some date-period, and when you build a report for a period, the "end date" should actually be something like Oct 31 2015 23:59:59 so your report works correctly - including all the data from last day of month.

I.e. you actually get the "last moment of the month" here. Not Last day.

OK, I'm going to shut up now.

Python Datetime - Exercises, Practice, Solution, Python datetime [52 exercises with solution] Write a Python program to get the number of days of a given month and year. Go to the editor. Get the last day of month in Java Java 8 Object Oriented Programming Programming The getActualMaximum() method returns the maximum value that the specified calendar field could have, based on the time value of this Calendar.


DateTime dCalcDate = DateTime.Now;
dtpFromEffDate.Value = new DateTime(dCalcDate.Year, dCalcDate.Month, 1);
dptToEffDate.Value = new DateTime(dCalcDate.Year, dCalcDate.Month, DateTime.DaysInMonth(dCalcDate.Year, dCalcDate.Month));

How to get the first and last date of current month using JavaScript , Given a month and the task is to determine the first and last day of that month in proper format using JavaScript. JavaScript getDate() Method: This method  ) // Returns a `YearMonth` object. .atEndOfMonth() // Determines the last day of the month for that particular year-month, and returns a `LocalDate` object. .toString() // Generate text representing the value of that `LocalDate` object using standard ISO 8601 format.


8.2. calendar — General calendar-related functions, Creates a Calendar object. firstweekday is an integer specifying the first day of the week. This iterator will return all days (as datetime.date objects) for the month and all the month or after the end of the month that are required to get a complete week. If l is given, it specifies the number of lines that each week will use. Therefore the more proper way to get the first and last date of the month would be this: var now = DateTime.UtcNow; var first = now.Date.AddDays(-(now.Date.Day - 1)); var last = first.AddMonths(1).AddTicks(-1); This way the original Kind of the DateTime instance is preserved.


Calculate last day of month with PHP and DateTime, Calculate last day of month with PHP and DateTime and DateTime. There are multiple options to calculate a the last (or first) day of a month. We only need to update that slightly to get the same result for any month: Serialize empty object as empty object instead of empty array with Symfony serializer. Improve this sample solution and post your code through Disqus. Previous: Write a program in C# Sharp to find the first day of the month against a given date. Next: Write a program in C# Sharp to find the First day of next month against a given date.


Get tuple of the first and last days of the last N months, I am trying to get a tuple of date (or datetime ) objects over the last N months. My thought was to use the dateutil package with something like this: def  First we create a Calendar Object and set it’s time with a Date Object. Next we can set the Day of the Month to the ‘actual minimum’. You can also just set this value to 1 because every month will start with 1. Finally we return a new Date Object representing the First day of the Month.