Problem
a value in a user interface field
If I were to use a timer, I could say
var maxDay = dtpAttendance.MaxDate.Day;
However, I’m attempting to obtain it via a DateTime object. So, if I’m in possession of this…
DateTime dt = DateTime.today;
What is the best way to get the first and last day of the month from dt?
Asked by CAD
Solution #1
A single value is stored in a DateTime structure, not a range of values. MinValue and MaxValue are static fields that store the range of possible values for DateTime structure instances. These fields are static and have nothing to do with a specific DateTime instance. They have something to do with the DateTime type.
static (C# Reference) is a recommended read.
UPDATE: I’ve figured out how to get a month range:
DateTime date = ...
var firstDayOfMonth = new DateTime(date.Year, date.Month, 1);
var lastDayOfMonth = firstDayOfMonth.AddMonths(1).AddDays(-1);
Answered by Sergey Berezovskiy
Solution #2
This is more of a lengthy response to @Sergey and @Steffen’s responses. After writing comparable code in the past, I chose to see what was the most performant while keeping in mind that clarity is also vital.
Here’s an example of a test run with ten 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()
I ran the tests with compiler optimization enabled in LINQPad 4 (in C# Program mode). For clarity and simplicity, I’ve factored the tested code as Extension methods:
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();
}
I was surprised by some of these results.
In most iterations of the test, FirstDayOfMonth AddMethod was slightly faster than FirstDayOfMonth NewMethod, despite the fact that there isn’t much in it. However, I believe that the latter has a slightly clearer aim, thus I prefer it.
LastDayOfMonth AddMethod was a clear loser against LastDayOfMonth AddMethodWithDaysInMonth, LastDayOfMonth NewMethod, and LastDayOfMonth NewMethodWithReuseOfExtMethodWithReuseOfExtMethodWithReuseOfExtMethodWithReuseOfExtMethodWithReuseOf There isn’t much difference between the three fastest, so it boils down to personal opinion. LastDayOfMonth NewMethodWithReuseOfExtMethod appealed to me because of its clarity and reuse of another valuable extension method. Its goal is clearer, in my opinion, and I am willing to overlook the minor performance penalty.
In the specific scenario where you may have previously calculated the first of the month, LastDayOfMonth SpecialCase assumes you are providing the first of the month and uses the add function with DateTime.DaysInMonth to get the result. This is faster than the other forms, as you’d expect, but I don’t see the sense of having this special instance in your arsenal unless you’re in severe need of speed.
Here’s an extension method class containing my recommendations, which I believe are in general accord with @Steffen’s:
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());
}
}
Thank you for taking the time to read this far! It’s been a lot of fun:). Please comment if you have any other suggestions for these algorithms.
Answered by WooWaaBob
Solution #3
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));
Answered by Steffen Mangold
Solution #4
The phrase “last day of the month” actually refers to the “first day of the *next* month, minus one.” So, instead of using the “DaysInMonth” technique, here’s what I do:
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 used AddMinutes(-1) instead of AddDays(-1) here is that you usually need these date functions for reporting for some date-period, and when you build a report for a period, the “end date” should be something like Oct 31 2015 23:59:59 so your report works correctly – including all the data from the last day of the month.
You receive the “final moment of the month” here, in other words. This isn’t the last day.
Okay, I’m going to stop talking now.
Answered by jazzcat
Solution #5
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));
Answered by sphaze
Post is based on https://stackoverflow.com/questions/24245523/getting-the-first-and-last-day-of-a-month-using-a-given-datetime-object