Develop Reference

How to set default month and day in the datetimepicker?

I have searched online and i only managed to find codes to set the year month and day.
dateTimePicker2.Value = new DateTime(2017,12,31);
I tried using the custom format and it does not seem to work
dateTimePicker2.CustomFormat = "DD/MM";
dateTimePicker2.Value = new DateTime(12,31);

You cannot create DateTime object only from day and month. DateTime simply doesn't have this kind of constructor. DateTime Constructors
So you need to go with some kind of "workaround"
- Use "dummy" year and when you need to use a date - use only Month and Day properties.
var dummyYear = 2000;
dateTimePicker2.Value = new DateTime(dummyYear, 12, 31);
Another workaround will be to use ParseExact method which will create DateTime based on the format you are using "dd/MM"
var date = DateTime.ParseExact("31/12", "dd/MM", CultureInfo.InvariantCulture);
dateTimePicker2.Value = date; // 12/31/2017
Notice that when you did not provide a year - current year will be used.
Another notice: DD is invalid format for days it should be lower case "dd"

You cannot partially set the date without a year, it's not valid.
What you cand do is specify the month and date in code as "default" values, and get the current year programmtically (or whatever year you want), and use that value for the year.

dateTimePicker1.Format = DateTimePickerFormat.Custom;
dateTimePicker1.CustomFormat = "dd/MM";
dateTimePicker1.Value = DateTime.Now;

Calculate Last Day of the Next Month

I am attempting to use the DateTime function in C# to calculate the last day of next month.
For example, today is December 17th 2015. I want the DateTime function to return January 31st 2016 (the last day of next month).
I am using the following to calculate the first day of next month (this works):
DateTime firstDayNextMonth = DateTime.Today.AddDays(-DateTime.Now.Day+1).AddMonths(1);

DateTime reference = DateTime.Now;
DateTime firstDayThisMonth = new DateTime(reference.Year, reference.Month, 1);
DateTime firstDayPlusTwoMonths = firstDayThisMonth.AddMonths(2);
DateTime lastDayNextMonth = firstDayPlusTwoMonths.AddDays(-1);
DateTime endOfLastDayNextMonth = firstDayPlusTwoMonths.AddTicks(-1);
Demo: http://rextester.com/AKDI52378

//system date or any date u want this case it is a calendar picker - 22/03/2016
DateTime today = dtpFrom.Value;
//Add a month to your date example , it now becomes - 22/04/2016
DateTime endOfMonth = new DateTime(today.Year, today.Month,today.Day).AddMonths(1);
//Get the last date off the above which is - 30
int getlastday = DateTime.DaysInMonth(endOfMonth.Year, endOfMonth.Month);
//Now set the date to the value which will be the last day off the next month - 30/04/2016
DateTime newDate = new DateTime(endOfMonth.Year, endOfMonth.Month, getlastday);

DateTime.DaysInMonth(DateTime.Now.AddMonths(1).Year, DateTime.Now.AddMonths(1).Month);

var lastDayInNextMonth = DateTime.DaysInMonth(DateTime.Now.AddMonths(1).Year, DateTime.Now.AddMonths(1).Month );
# Ben : DateTime.Now.AddMonths(1) will add 1 month to the current date not substract 11 months.
DateTime.Now.AddMonths(1).Year will give 2016 not 2015 refer the attached image

try this:
int Day= DateTime.DaysInMonth(DateTime.Now.Year, DateTime.Now.Month+1>12 ? 01 : DateTime.Now.Month+1 );

String To DateFormat Conversion

Is there any way I don't have to specify the number of digits in day/month/year?
For e.g 1/2/1991
I want a method which satisfies both 1/2/1991,11/3/1990,12/12/1991
I don't know how many digits will be there in either month, year, or days.
My code is
string copy = splittedData[0] + splittedData[1] + splittedData[2];//date+month+year
DateTime datetime = DateTime.ParseExact(copy, "ddMMyyyy", CultureInfo.InvariantCulture);
DateTime dateAndTime = datetime;
The problem is the number of digits in splitted data array are not known to me and thus the above format "ddMMyyyy" give me exception on some cases.

Since you already have the day month and year then just create a date with the three of them like so;
DateTime date = new DateTime(year, month, day);
No parsing is necessary. You already have all the fields you want to create the date, and you dont need to put it into a special format to create a date.
If you are not sure the if the input is valid, then wrap the creation in a try/catch block to catch an ArgumentOutOfRangeException should it should occur.

Since you updated your question with the code you have, you can concatenate date components with a separator like:
string copy = splittedData[0] + "/" + splittedData[1] + "/" + splittedData[2];
Later you can do:
DateTime dt = DateTime.ParseExact(copy, "d/M/yyyy", CultureInfo.InvariantCulture);
I used the format "d/M/yyyy" with single d and M which would account for both single/double digit day/month.
So it will work for dates like:
01/01/2013
1/01/2013
22/09/2013
02/9/2013

DateTime.ParseExact is specifically intended to not allow what you are asking for. DateTime.Parse will allow it, though.
You say you have the 3 parts as separate strings -- if you insert the /'s and parse, it should succeed (InvariantCulture expects the order month-day-year):
string datetimeString = string.Join("/", new[] {month, day, year});
DateTime datetime = DateTime.Parse(datetimeString, CultureInfo.InvariantCulture);
Or you could convert them to integers and construct a DateTime directly:
DateTime datetime = new DateTime(Convert.ToInt32(year), Convert.ToInt32(month), Convert.ToInt32(day));

What #n00b said. You've already got the individual components of the date: why are you globbing them back together just so you can call DateTime parsing routines? Just do something like this:
private static DateTime StringToDateTime( string year , string month , string day )
{
int yyyy = int.Parse(year) ;
int mm = int.Parse(month) ;
int dd = int.Parse(day) ;
DateTime dt = new DateTime(yyyy,mm,dd) ;
return dt ;
}
As an added bonus, The above code will probably run faster than DateTime.Parse() or DateTime.ParseExact().

Get the system date and split day, month and year

My system date format is dd-MM-yyyy(20-10-2012) and I'm getting the date and using separator to split the date, month and year. I need to convert date format (dd/MM/yyyy) whether the formats returns at any date format.
string sDate = string.Empty;
DateTime _date = DateTime.Now;
int count = 0;
string format = "dd-MM-yyyy";
sDate = dateFormat.ToString(format);
string[] Words = sDate.Split(new char[] { '-' });
foreach (string Word in Words)
{
count += 1;
if (count == 1) { Day = Word; }
if (count == 2) { Month = Word; }
if (count == 3) { Year = Word; }
}

Here is what you are looking for:
String sDate = DateTime.Now.ToString();
DateTime datevalue = (Convert.ToDateTime(sDate.ToString()));
String dy = datevalue.Day.ToString();
String mn = datevalue.Month.ToString();
String yy = datevalue.Year.ToString();
OR
Alternatively, you can use split function to split string date into day, month and year here.
Hope, it will helps you... Cheers. !!

You can do like follow:
String date = DateTime.Now.Date.ToString();
String Month = DateTime.Now.Month.ToString();
String Year = DateTime.Now.Year.ToString();
On the place of datetime you can use your column..

You can split date month year from current date as follows:
DateTime todaysDate = DateTime.Now.Date;
Day:
int day = todaysDate.Day;
Month:
int month = todaysDate.Month;
Year:
int year = todaysDate.Year;

You should use DateTime.TryParseExcact if you know the format, or if not and want to use the system settings DateTime.TryParse. And to print the date,DateTime.ToString with the right format in the argument. To get the year, month or day you have DateTime.Year, DateTime.Month or DateTime.Day.
See DateTime Structures in MSDN for additional references.

Without opening an IDE to check my brain works properly for syntax at this time of day...
If you simply want the date in a particular format you can use DateTime's .ToString(string format). There are a number of examples of standard and custom formatting strings if you follow that link.
So
DateTime _date = DateTime.Now;
var _dateString = _date.ToString("dd/MM/yyyy");
would give you the date as a string in the format you request.

TL&DR; The DateTime Object in C# has the ability to extract the string information using the ToString() function with the correct format string. (ie: "yyyy-mm-dd");
string onlyDateWithoutTimeStamp = lastDate.ToString("yyyy-MM-dd");

Replace date in a DateTime with another date

How do you change the day part of a DateTime structure?
I have a DateTime with the value of 3/31/2009 8:00:00 AM and would like to change it to any other date but the time should be same. I don't need a solution to add day(s), I need to replace it with any arbitrary date.
How do I achieve this?

To construct a DateTime from two other DateTimes, one of which contains the date and one of which contains the time, you can use:
DateTime result = date.Date + time.TimeOfDay;

day = 1
month = 4
year = 2009
Date2 = new DateTime(day,month,year,Date1.Hours,Date1.Minute,Date1.Seconds)

s = s.AddDays(1)

You should use DateTime.AddDays method. Like this:
DateTime s2 = s.AddDays( 1 );