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);
Related
This question already has answers here:
Get the correct week number of a given date
(20 answers)
Closed 4 years ago.
I am trying to get the week number from a date time and in my case, the first day of the week is Monday and I want to follow the FirstFourDays convention.
To check the results, I am checking this webpage:
https://espanol.epochconverter.com/semanas/2020
To get the week number, I using the method:
System.Globalization.CultureInfo.InvariantCulture.Calendar.GetWeekOfYear();
So I am trying to get the week number of the date 2019-12-29, so I use this code:
System.Globalization.CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(new DateTime(2019, 12, 29), System.Globalization.CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
The result is week 52. It is correct.
Now I am trying to get the week number of the 2019-12-30, the week number that I get is 53, it is wrong, because 2019 has only 52 weeks. In fact, 2019-12-30 belongs to the same week than 2020-01-01, that it is week 1, that is correct, so I don't understand why I can get two different results for the same date.
How I could get the correct result always? Or how would it be the correct way to get the week number of any date?
There's a blog article explaining this behavior and proposing a solution.
The issue:
Several people have noticed that Calendar.GetWeekOfYear() is almost like the ISO 8601 week when passed CalendarWeekRule.FirstFourDayWeek and DayOfWeek.Monday, however it is a little bit different. Specifically ISO 8601 always has 7 day weeks. If the first partial week of a year doesn't contain Thursday, then it is counted as the last week of the previous year. Likewise, if the last week of the previous year doesn't contain Thursday then its treated like the first week of the next year. GetWeekOfYear() has the first behavior, but not the second.
The proposed solution would be this:
A simple workaround to consistently get the ISO 8601 week is to realize that consecutive days Monday through Sunday in ISO 8601 weeks all have the same week #. So Monday has the same week # as Thursday. Since Thursday is the critical day for determining when the week starts each year my solution is to add 3 days if the day is Monday, Tuesday or Wednesday. The adjusted days are still in the same week, and use values that GetWeekOfYear and ISO 8601 agree on.
// This presumes that weeks start with Monday.
// Week 1 is the 1st week of the year with a Thursday in it.
public static int GetIso8601WeekOfYear(DateTime time)
{
// Seriously cheat. If its Monday, Tuesday or Wednesday, then it'll
// be the same week# as whatever Thursday, Friday or Saturday are,
// and we always get those right
DayOfWeek day = cal.GetDayOfWeek(time);
if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)
{
time = time.AddDays(3);
}
// Return the week of our adjusted day
return cal.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);
}
All credits for this go to Shawn Steele.
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
I am facing issue while converting datetime to UTC.
User enters date in mydate in the webform, which we need to convert in UTC and store it in DB.
User also selects timezone from the list which is stored in selectedTimeZone variable. So we have to convert mydate from selectedTimeZone to UTC.
TimeZoneInfo.ConvertTimeToUtc(mydate, selectedTimeZone);
Example#1 :
If mydate = 05/02/2016 09:00 AM and selectedTimeZone = EST (-5:00) then
TimeZoneInfo.ConvertTimeToUtc(mydate, selectedTimeZone) returns
05/02/2016 13:00
which is off by one hour
Example#2
If mydate = 05/02/2016 09:00 AM and selectedTimeZone = IST (indian
standard time) (+5:30) then TimeZoneInfo.ConvertTimeToUtc(mydate,
selectedTimeZone) returns 05/02/2016 03:30
which is correct
There are multiple examples like this.
What is the issue?
Edit:
I don't need to convert user input to DateTime as .net does it, we are getting mydate in mvc action method parameter.
I tried it by setting local timezone of a machine to UTC, London, IST..... but it makes no difference to the output.
It seems adjustment rules for given timezone play their role here. Take the following code:
var mydate = new DateTime(2016, 05, 02, 9, 0, 0);
var selectedTimeZone = TimeZoneInfo.FindSystemTimeZoneById("Eastern Standard Time");
var dstRule = selectedTimeZone.GetAdjustmentRules().FirstOrDefault(c => c.DateStart < mydate && c.DateEnd > mydate);
There is just one adjustment rule for EST timezone which is active at given time (it is active starting at 2007 year), and this rule works from second Sunday of March to first Sunday of November every year, adjusting time by one hour during that period. That is why you observe that behaviour. If you try to convert date outside of period for this rule (say, in February) - you will get what you would expect.
So to clarify: EST timezone has base offset of UTC-5, but because of daylight savings becomes UTC-4 during summer, and because your time is "summer" - .NET actually correctly converts that to UTC, it's not "off by one hour".
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.
This question already has answers here:
DateTime difference in days on the basis of Date only
(3 answers)
Closed 8 years ago.
I'm using ASP.NET, C# and SQL Server. In the database the date is: 2014-10-28. Just year, month and day, no time portion.
I have this code:
DateTime data1 = new DateTime();
DateTime data2 = DateTime.Now;
data1 = reader.GetDateTime(3);
double total= (data2 - data1).TotalDays;
Response.Write(total.ToString());
The only problem of this is the output. The output of this is "4,81351624131366". Because probably this is the difference of the hours. Its possible to set the data2 to give me just the Y,M and D?
Or convert the total into days?
The property TotalDays returns the fractional portion of days too. Since the date you're comparing was 4 days ago at midnight, you're getting 4 full days and an additional 4/5th of a day (since it's not exactly midnight when you're executing this).
Either what Jon said (use DateTime.Today instead of DateTime.Now to get the date portion with a time of midnight), or use the Days property, which will just return a rounded 4 and drops the fractional portion.