Develop Reference

MM-dd-yyyy to Julian yyddd

I am trying to get a label to display the Julian date in a specific format. The last two digits of the year then the day in the year, so for example January 1 2021 would be 210001. I am having difficulty getting it to display both of these values attached and making the day slot have 3 values instead of 2.
This is what I have.
This just gives the day of the year but still not as a 3 digit day so it shows 1 as 1 instead of 001 which is my goal
Any help would be appreciated! :)

Unfortunately the date time formats do not include anything for day of the year so you'll have to create this yourself. You can format a number to have leading zeros using the D format where you specify the length you want. You can however use the date formats to get the last two digits of the year. So the following should give you the desired formatted string for a date.
public string ToYYJJJ(DateTime date)
{
return date.ToString("yy") + date.DayOfYear.ToString("D3");
}

C# Adding years and days in a single DateTime variable

Am I missing something simple?
I am trying to calculate a date 17 years and 364 days before the given date.
Is there a way to do this without converting everything into days? I am trying to avoid dealing with leap years. I am doing the following:
DateTime date = Convert.ToDateTime(tId2);
string tId4a = Convert.ToString(tId4);
var age1 = tId4a.Substring(0, 2);
int age2 = Convert.ToInt32(age1) - 1;
DateTime sub1 = date.AddYears(-age2);
I was hoping to do something simple like:
DateTime sub1 = date.AddYears(-age2) + date.AddDays(-364);
I am being told that I cannot use the '+' in the DateTime.
Sorry, but I am new to this. The reason the age2 variable is used is because at times that value will change. But, the 364 should be consistent. I am creating something to test a date boundary.
Did I overlook something simple?
Thanks.

What you do is you add the "date age2 years ago" to the "date 364 days ago".
Instead do this:
DateTime sub1 = date.AddYears(-age2).AddDays(-364)
This at first subtracts the years and then subtracts the days from the resulting value.

You can't add dates, but you can certainly chain method calls together
date.AddYears(-age2).AddDays(-364);
This is for all intents and purposes the same thing as trying to add them together.

It really sounds like you want to go with tid4 years ago, but go to the next day after that.
The way you are doing it, is that you subtract 1 from that to get age2. Then you subtract that many years, and you also subtract 364 days from your date. This will be more sensitive to leap years. If the resulting date happens to be between Jan 1 and Feb 28 of a leap year, you will end up with one day later than you wanted.
364 is a very suspect number. I tend to think you are using that to mean "the number of days in a year minus one". But the number of days in a year is not always 365. In leap years, the number of days is 366. In such years, subtracting 364 is not 1 day less than a year. It is actually 2 days less than a year, so you would be off.
What you really should do, if I am reading you correct, is to just subtract the number of years, then add one day back in.
DateTime date = Convert.ToDateTime(tId2);
string tId4a = Convert.ToString(tId4);
int age = Convert.ToInt32(tId4a.Substring(0, 2))
DateTime sub1 = date.AddYears(-age).AddDays(1);

I think that it is valuable to mention that DateTime is an object, and that .AddYears(), .AddDays(), etc all return a new DateTime object which is why you cannot add them together like primitive types. So when you run:
DateTime sub1 = date.AddYears(-age2).AddDays(-364);
date.AddYears(-age2) returns a new object, and then .AddDays(-364) is using the new DateTime object and not the date instance.
For more info:
https://msdn.microsoft.com/en-us/library/system.datetime(v=vs.110).aspx

How to remove time in date time?

How to remove a time in date time ? on column date its only display format
I store the value on repository combobox dropdown, and it store the value including the time. How do I remove the time?
I know there's so many question about this. But the solution was by converting it into a date.tostring("dd MMM yyyy"). Is there a solution beside convert it into string? I want the value was date time not a conversion of string.
The code I am using still giving me a time.
DateTime date = Convert.ToDateTime(gridView1.GetDataRow(i)["date"]);

You just forgot to specify the date at the end of the conversion
DateTime date = Convert.ToDateTime(gridView1.GetDataRow(i)["date"]).Date;

DateTime as the name implify, stores date and time.
You cannot remove time part from date because time is an integral part of date.
To understand this you will have to understand how the date and time are stored. Internally, the date and time is stored as a rational number (in fractions). In computer system 24 hours are considered as numeric 1, so when your value is increased by 1 that means your date is increased by 1 day. If the value is increased by 0.5 that means your date is increased by 12 hours (half day).
So, when you have value 42613.00 that means 31st August at midnight (just when the day started) and if you have value 42613.25 that means 6 AM of 31 Aug 2016 and 42613.50 means 12 noon of 31 Aug 2016 (and 42613.39236 means 9:25:00 AM of 31 Aug 2016)
The smallest fraction of time that need to be stored is 1 millisecond. That means the values of DateTime field should have a precision of more than 0.0000000115740740740741. But this is an irrational value (in binary) and hence cannot be stored as such (the nearest match is 1.00000000000000000000000000110001101101011101010000111010111111..., ... means there are more), so I can say that milliseconds are to their nearest approximation values.
.
That said,
if you wish to take only Date part, you can create your own class or struct to store date part of the DateTime and then override operators for date arithematic and provide implicit conversions to convert them to DateTime if any code that expect DateTime field.

Get full date when DateTime.DayOfYear and Year is given

Is there a possibility to get the full date out of the year and the dayOfYear value?
For example: today is Thursday, 19th of february 2015.
The dayOfYear-Value of today is 50.
If I have the value 75 and the year 2010, how am I able to get the matching date?
It could be displayed in a textBox, dateTimePicker, whatever.
But you only have the information year & dayOfYear.
Thanks

You can use the following code:
DateTime day = New DateTime(2010, 1, 1).AddDays(75 - 1);
First get the first day of the year, then add necessary day count minus one (you are already on the first date of the year) days to the first day and you are there.

C# DateTime Math - Crossing into Previous Months

I just discovered a DateTime math bug in my code today, August 1st, the start of a new month.
I am setting a DateTime class member to the DateTime value of 7 days ago. So the value it should have on August 1st, would be July 25th
I clearly see the problem when trying to subtract 7 from 1 (DateTime.Now.Day-7).
So how do I create a DateTime object that always works regardless of what day of the month it is?
Error:
Year, Month, and Day parameters describe an un-representable DateTime.
Here is the code:
DateFrom = (new DateTime(DateTime.Now.Year, DateTime.Now.Month, DateTime.Now.Day-7));
What is the correct way to set this member to be 7 days ago from current DateTime?

Just subtract seven days from DateTime.Now:
DateFrom = DateTime.Now.Date.AddDays(-7);

DateFrom date = DateTime.Now.AddDays(-7);

You can use the AddDays method of datetime object that takes integer. As it takes negative values as well so you can easily move back in time.
DateFrom = DateTime.Now.AddDays(-7);