Develop Reference

Nodatime and days in month

I'm trying to get the number of days in a month. So I have the following
var fromdate = LocalDateTime.FromDateTime(DateTime.Now).Date;
Now I just need the number of days in that LocalDateTime variable that I just created. It seems like I could enter:
var m = fromdate.Calendar.GetDaysInMonth(year, month)
But I already know the year/month in fromdate. What is it that I'm doing wrong?

You can use DateTime's GetDaysInMonth method:
int days = DateTime.DaysInMonth(year, month);

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 );

Calculate the annual use of a service starting from the date of signing

I need to calculate the annual use of a service starting from the date of signing. Something like:
select Count(*) from TABLENAME where Date >= MYDATE
MYDATE need to be calculate from a subscription date and I need to get the last year date from subscription referring to the current date
Some examples:
subscription date: 2007-06-29
if current date is : 2015-04-29 then date is: 2014-06-29
if current date is : 2015-06-29 then date is: 2015-06-29
if current date is : 2015-06-29 then date is: 2015-06-29
I'm using c# to calculate the date but it crashes in leapyear:
var date = new DateTime(DateTime.Now.Year, subscriptionDate.Month, subscriptionDate.Day);
if (DateTime.Now.Date < date)
{
date = date.AddYears(-1);
}
I was wondering if there were a clever/better way to do it in c# or mysql also handling leapyear
---UPDATE----
Running example with suggested solutions

Well, I'd do it in Noda Time, myself:
LocalDate subscriptionDate = ...;
LocalDate today = ...; // Need to take time zone into account
int years = Period.Between(subscriptionDate, today);
return subscription.PlusYears(years);
With .NET that would be slightly harder, but I'd still go for the approach of adding years (and letting it do the truncation for Feb 29th):
// Only call this *once* - otherwise you could get inconsistent results
DateTime today = DateTime.Today;
int years = today.Year - subscriptionDate.Year;
DateTime candidate = subscriptionDate.AddYears(years);
// We might have overshot, in which case lower the number of years.
return candidate <= today ? candidate : subscriptionDate.AddYears(years - 1);

Thanks to Yuri Dorokhov answer and Jon Skeet suggestion
I found a solution that works well and handle leap year:
int year = DateTime.Now.DayOfYear >= subscriptionDate.DayOfYear ?
DateTime.Now.Year : DateTime.Now.Year - 1;
var date = new DateTime(year, 1, 1).AddDays(subscriptionDate.DayOfYear - 1);
--------UPDATE------
I leave here this answer as reference but it does not handle well leap year so don't use it

Use mysql DATE_SUB function
DATE_SUB(Date, INTERVAL 1 YEAR)

How do you convert an int representing days-from-zero to DateTime?

I have an int representing a number of Gregorian days from Year Zero (thanks, Erlang). How do I convert this to a DateTime object? I can't create a DateTime(0,0,0), and Convert.DateTime(int) throws an invalid cast.

If you have a number, and you know the date that it represents (from Erlang), you can calculate the offset from any date you choose. Preferred is a base date in the zone that the results will be in, this will minimize calender conversion effects. (The Gregorian calendar is valid from about 1600).
If you know that offset, you can use the choosen date as the base for future calculations.
Example:
I want my offset date to be: 1/1/2000. This will be the date that I calculcate from.
I know number 37892 from erlang is actually 1/1/1970 (this is an example).
Then I can calculate the offset:
var myBaseDate = new DateTime(2000,1,1);
var exampleNrOfDays = 37892;
var exampleDate = new DateTime(1970,1,1);
var offset = exampleDate - myBaseDate;
var offsetInDays = exampleNrOfDays - (int)offset.TotalDays;
// Now I can calculate
var daysFromErlang = 30000; // <= example
var theDate = myBaseDate.AddDays(daysFromErlang - offsetInDays);

This shows how to calculate number of days from a given date. http://dotnetperls.com/datetime-elapsed

if day zero is 0/0/0 then it is 365+30+1 day before DateTime.Min which is 1/1/1. So you can subtract days from year zero by 365+30+1 and add to DateTime.Min
Now Month 1 is January which is 31 days but what is Month 0? I assumed it is 30 days.

With 0, you probably mean 0:00 on the 1st of January, year 1. There is no year 0 in the gregorian calendar as far as i know.
If the above is right, you can just do
DateTime date = new DateTime();
date.AddDays(numberOfDays);
because the default constructor 'DateTime()' returns the "zero" DateTime object.
See the DateTime reference for more informations.

I am not sure if you are aware of this, but there is a Calendar object in System.Globalization. Not only that but there is a GregorianCalendar object as well.
so try this:
GregorianCalendar calendar = new GregorianCalendar();
DateTime minSupportedDateTime = calendar.MinSupportedDateTime;
//which is the first moment of January 1, 0001 C.E.
DateTime myDate = minSupportedDateTime.AddDays(55000);
//this is when you add the number of days you have.
Thanks,
Bleepzter
PS. Don't forget to mark my answer if it has helped you solve your problem! Thanks.

DateTime Object Representing Day of Week

How can I convert a number between 1 and 7 into a DateTime object in C# which represents the day of the week? The numbers are coming from a XML file which I am parsing. I am retrieving each instance of a field containing a number between 1 and 7 which represents a day of the week between Sunday and Saturday.

I would assume casting to a DayOfWeek object would give you a day of the week
DayOfWeek day = (DayOfWeek)myInt;
As far as a DateTime object goes, the object represents a specific day, not necessarily a random day of the week. You may try adding a # of days to a specific date if this is what you're trying to achieve.
http://msdn.microsoft.com/en-us/library/system.dayofweek.aspx

In order to get a DateTime, you'd need a specific range of dates that you want the weekday to fall under (since a DateTime is a specific date and time, and a weekday isn't).
There is a DayOfWeek enumeration (whose values actually range from 0-6). If all you need is something to represent the day of the week, then you should be able to cast your int to a DayOfWeek like..
DayOfWeek myDay = (DayOfWeek)yourInt;
If you need an actual DateTime, you'll need a start date. You could then do...
DateTime myDate = startDate.AddDays(
(int)startDate.DayOfWeek >= yourInt ?
(int)startDate.DayOfWeek - yourInt :
(int)startDate.DayOfWeek - yourInt + 7);
This will give you a DateTime for the next occuring instance of the day of the week you're describing.

DayOfWeek.Sunday is zero, so you could start with an arbitrary fixed date that you know to be Sunday, and add a value between 0 and 6:
public DateTime GetDayOfWeek(int dayOfWeek)
{
if (dayOfWeek < 0 || dayOfWeek > 6) throw new ArgumentOutOfRangeException(...);
// 4 January 2009 was a Sunday
return new DateTime(2009,1,4).AddDays(dayOfWeek);
}
I'm not sure why you would want this though.
If you only want it to get a localized version of the day of the week as in:
GetDayOfWeek(3).ToString("dddd"); // Gets name of day of week for current culture
an alternative would be to use DateTimeFormatInfo.DayNames or DateTimeFormatInfo.AbbreviatedDayNames for the culture you want.

A DateTime instance represents alway a complete date and cannot only represent a day of the week. If the actual date does not matter, take any monday (assuming 0 represents monday) and just add the number of the day.
Int32 dayOfWeek = 3;
// date represents a thursday since 2009/04/20 is a monday
DateTime date = new DateTime(2009, 04, 20).AddDays(dayOfWeek);
Else I agree with Adam Robinson's answer - if you just want to hold the day of a week, stick with the DayOfWeek enum (zero is sunday) instead of using an integer.