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

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);

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.

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));

"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.

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));

Here you can add one month for the first day of current month than delete 1 day from that day.

DateTime now = DateTime.Now;
var startDate = new DateTime(now.Year, now.Month, 1);
var endDate = startDate.AddMonths(1).AddDays(-1);

If you only care about the date

var firstDay = new DateTime(date.Year, date.Month, 1, 0, 0, 0, date.Kind);
var lastDay = new DateTime(date.Year, date.Month, 1, 0, 0, 0, date.Kind).AddMonths(1).AddDays(-1);
        

If you want to preserve time

var firstDay = new DateTime(date.Year, date.Month, 1, date.Hour, date.Minute, date.Second, date.Kind);
var lastDay = new DateTime(date.Year, date.Month, 1, date.Hour, date.Minute, date.Second, date.Kind).AddMonths(1).AddDays(-1);

Try this one:

string strDate = DateTime.Now.ToString("MM/01/yyyy");

The accepted answer here does not take into account the Kind of the DateTime instance. For example if your original DateTime instance was a UTC Kind then by making a new DateTime instance you will be making an Unknown Kind instance which will then be treated as local time based on server settings. 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.

For Persian culture

PersianCalendar pc = new PersianCalendar();            
   
var today = pc.GetDayOfMonth(DateTime.Now);
var firstDayOfMonth = pc.GetDayOfMonth(DateTime.Now.AddDays(-(today-1)));
var lastDayOfMonth = pc.GetDayOfMonth(DateTime.Now.AddMonths(1).AddDays(-today));            
Console.WriteLine("First day "+ firstDayOfMonth);
Console.WriteLine("Last day " + lastDayOfMonth);

I used this in my script(works for me) but I needed a full date without the need of trimming it to only the date and no time.

public DateTime GetLastDayOfTheMonth()
{
    int daysFromNow = DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month) - (int)DateTime.Now.Day;
    return DateTime.Now.AddDays(daysFromNow);
}

You can do it

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

Give this a try. It basically calculates the number of days that has passed on DateTime.Now, then subtracts one from that and uses the new value to find the first of the current month. From there it uses that DateTime and uses .AddMonths(-1) to get the first of the previous month.

Getting the last day of last month does basically the same thing except it adds one to number of days in the month and subtracts that value from DateTime.Now.AddDays, giving you the last day of the previous month.

int NumberofDays = DateTime.Now.Day;
int FirstDay = NumberofDays - 1;
int LastDay = NumberofDays + 1;
DateTime FirstofThisMonth = DateTime.Now.AddDays(-FirstDay);
DateTime LastDayOfLastMonth = DateTime.Now.AddDays(-LastDay);
DateTime CheckLastMonth = FirstofThisMonth.AddMonths(-1);

You can try this for get current month first day;

DateTime.Now.AddDays(-(DateTime.Now.Day-1))

and assign it a value.

Like this:

dateEndEdit.EditValue = DateTime.Now;
dateStartEdit.EditValue = DateTime.Now.AddDays(-(DateTime.Now.Day-1));

Create an instance of DateTime class

DateTime dateTime = DateTime.Now;

If you want to get the last day of the month you can do this

int lastDayOfMonth = DateTime.DaysInMonth(caducidadPuntos.Year, caducidadPuntos.Month);

If you want to get the first day of the month, you can do this

DateTime firstDayMonth = new DateTime(dateTime.Year, dateTime.Month, 1);

We had the requirement of being able to get the start and end of a given dates month, including times, inclusively. We ended up utilizing the aforementioned solutions, huge thanks to everyone here, and combined it into a util class to be able to get the start and end for a given month and year number combination up to the last millisecond. Including what we moved forward with in the event it helps someone else.

The util:

    public class DateUtil
    {
        public static (DateTime startOfMonth, DateTime endOfMonth) GetStartAndEndOfMonth(int month, int year)
        {
            DateTime startOfMonth = GetStartOfMonth(month, year);
            DateTime endOfMonth = GetEndOfMonth(month, year);
            return (startOfMonth, endOfMonth);
        }

        public static DateTime GetStartOfMonth(int month, int year)
        {
            return new DateTime(year, month, 1).Date;
        }

        public static DateTime GetEndOfMonth(int month, int year)
        {
            return new DateTime(year, month, 1).Date.AddMonths(1).AddMilliseconds(-1);
        }
    }

Usage:

(DateTime startOfMonth, DateTime endOfMonth) = DateUtil.GetStartAndEndOfMonth(2, 2021); // February, 2021

easy way to do it

Begin = new DateTime(DateTime.Now.Year, DateTime.Now.Month,1).ToShortDateString();
End = new DataFim.Text = new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month)).ToShortDateString();

DateTime dCalcDate = DateTime.Now;
var startDate = new DateTime(Convert.ToInt32(Year), Convert.ToInt32(Month), 1);
var endDate = new DateTime(Convert.ToInt32(Year), Convert.ToInt32(Month), DateTime.DaysInMonth((Convert.ToInt32(Year)), Convert.ToInt32(Month)));