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c# Is it possible to subtract a day in the datetime format

I have the following simple example:
string dt = DateTime.Now.ToString("yyyy-MM-dd hh.mm.ss");
I can't change the DateTime.Now, but I can change datetime format yyyy-MM-dd hh.mm.ss. Following this example the result must be today's date, but I need to get yesterday's date with the same parameters except day (year, month, hours, minutes and seconds). E.g. 2015-08-23 12.09.59 must be 2015-08-22 12.09.59. So is it possible to use some "-" operator or something else inside the datetime format to achieve the result?

If you want yesterday's date, you can do this
string dt = DateTime.Now.AddDays(-1).ToString("yyyy-MM-dd hh.mm.ss");
DateTime.AddDays() lets you add number of days, positive for future date, negative for past date.

E.g. 2015-08-23 12.09.59 must be 2015-08-22 12.09.59. So is it
possible to use some "-" operator or something else inside the
datetime format to achieve the result?
No, it's not possible inside the DateTime format. you can not change any thing. Because it is only for define format of the Date to display in string format. Any addition or subtraction can only be done before converting it to string format as suggested by "Arghya C".
Can you explain your limitation so we can solve your problem.

If you can only influence the date time pattern, than use the roundtrip format and parse the returning string back to a date time, add the calculation and format it into the desired format:
var dateTimeString = badLibrary.GetDateTime("o");
var dateTime = DateTime.Parse(dateTimeString, null, DateTimeStyles.RoundtripKind);
var newDateTime = dateTime.AddDays(-1);
return newDateTime.ToString("yyyy-MM-dd hh.mm.ss");

I need only date value part and time value part using ASP.NET C#

I have two parameters one for date and another for time, and i need date value part and time values part.
My two parameters are below.
// For Date parameter
DateTime dt = DateTime.ParseExact("01-jan-1999", "dd-MMM-yyyy", CultureInfo.InvariantCulture);
bo.Dateused5 = dt;
// For Time parameter
string Fromtiming = ddl_FromHours.SelectedItem.ToString() + ":" + ddl_FromMinutes.SelectedItem.ToString();
DateTime InterviewTime = Convert.ToDateTime(Fromtiming);//StartTime
bo.Dateused4 = InterviewTime;//InterviewTime
so i need to send mail to the candidate to only date part, should not contain time and time part, should not contain date.

are you looking for this:
DateTime dt = DateTime.ParseExact("01-jan-1999", "dd-MMM-yyyy", CultureInfo.InvariantCulture);
string mailDate = dt.ToString("dd-MMM-yyyy");// will give 01-jan-1999
string date = dt.ToString("dd-MM-yyyy"); // will give 01-01-1999
You can also try using String.Format()
string mailDate = String.Format("{0:dd-MM-yyyy}", dt); // will give 01-01-1999

You can use ToShortDateString():
DateTime dt = DateTime.ParseExact("01-jan-1999", "dd-MMM-yyyy", CultureInfo.InvariantCulture);
var date = dt.ToShortDateString();
Note that it uses date format attached to the current thread's culture info.

You would need to use strings rather than dates, so change the type of your variables to string so that
bo.Dateused5 = dt.ToString("dd-MMM-yyyy")
would set Dateused5 to a string of the date component, then
bo.Dateused4 = InterviewTime.ToString("HH:MM");
would set Dateused4 to the time component.

Couldn't test your code but I am very sure there are Functions "DateValue" and "TimeValue" you can make use of.
Something like,
Format(DateValue(any datetime), "dd-MM-yyyy")
gives you Only Date in the specified format. Similar way for TimeValue

Datetime conversion in C# - using formats

I need to convert a String to DateTime format, for this I just tried like
DateTime.ParseExact(DateOfBirth,"MM/dd/yyyy",CultureInfo.InvariantCulture);
it's working fine when I pass the value like 05/30/2012.
But if I try to pass the value as 5/30/2012 its showing error:
String was not recognized as a valid DateTime
To fix this I tried like
DateTime.ParseExact(String.Format("{0:MM/dd/yyyy}", DateOfBirth), "MM/dd/yyyy",
CultureInfo.InvariantCulture);
it's still not working. Here If I try String.Format("{0:MM/dd/yyyy}", DateOfBirth) for the value 5/30/2012 its showing the same format instead of 05/30/2012.
How can I fix this, can anyone help me here...
check this link
string to DateTime conversion in C#

Use M/d/yyyy instead of the format specifier you're using. Using only a single M matches months with leading zeros as well. This is also true for d.

assuming your DateOfBirth string is always separated by slashes, you could try something like this:
string[] dateParts = DateOfBirth.Split('/');
DateTime.ParseExact(string.Format("{0:00}", dateParts[0]) + "/" + string.Format("{0:00}", dateParts[1]) + "/" + string.Format("{0:0000}", dateParts[2]));
I think the issue is the format string can't be recognized since DateOfBirth is not a DateTime object. Thus, you enforce formatting by reformatting the string yourself

There is an overload which might be of your interest
DateTime.ParseExact(DateOfBirth,
new[] { "MM/dd/yyyy", "M/dd/yyyy" },
CultureInfo.InvariantCulture,
DateTimeStyles.None);
This should help you take care of single as well as two digit month part (since 5 is failing for you as the format is MM)

Since you have separators in your string (ie /), you can just do;
DateTime.ParseExact(DateOfBirth,"M/d/yyyy",CultureInfo.InvariantCulture);
That will parse either single or double digit days/months. When you use MM/dd/yyyy, you're requiring them both to be double digit numbers, and that's obviously not what you want in this case.

Try just "d" instead of "MM/dd/yyyy".
So, the statement should be:
var date = DateTime.ParseExact(DateOfBirth, "d", CultureInfo.InvariantCulture);
The documentation for this is here:
http://msdn.microsoft.com/en-us/library/w2sa9yss.aspx
Edit
Oops, I misread the documentation. It should be "M/d/yyyy".

In case you need to make it culture-independent..
var dateTimeFormat = CultureInfo.CreateSpecificCulture(CultureInfo.CurrentUICulture.Name).DateTimeFormat;
dateTimeFormat.ShortDatePattern =
Regex.Replace(Regex.Replace(dateTimeFormat.ShortDatePattern, "[M]+", "MM"), "[d]+", "dd");
var newDate = date.HasValue ? date.Value.DateTime.ToString("d", dateTimeFormat) : null;

How to get today's date in mm/dd/yyyy format into a datetime variable in c#

I want today's date in mm/dd/yyyy format from a DateTime variable. How to get it?
I need to check this with some date variable so it must be also in date variable format?plz help
Ex: i need to get today date in mm/dd/yyyy format and i already date which is datetime datatype in mm/dd/yyyy format and i have to compare them

You should use DateTime.Today:
DateTime today = DateTime.Today; // As DateTime
string s_today = today.ToString("MM/dd/yyyy"); // As String
Edit: You edited your post to add another question, so here comes my edit to supply at least some sort of answer.
Update While you can use DateTime.Compare() you should use plain comparisson:
if(today < otherdate)
{
// Do something.
}
Alternatively, you can use DateTime-variables to check against other DateTime-variables using the DateTime.Compare() method. Both otpions will work and it comes down to preference and what you want to do with the result.
int result = DateTime.Compare(today, otherdate);
if(result < 0)
MessageBox.Show("Today is earlier than the 'otherdate'");
elseif(result > 0)
MessageBox.Show("Today is later than the 'other date'");
else
MessageBox.Show("Dates are equal...");

string datestring = DateTime.Now.ToString("MM/dd/yyyy");

MSDN say: Custom Date and Time Format Strings

To convert DateTime variable to string in the specified format:
DateTime d = ...;
string s = d.ToString("MM/dd/yyyy");
If you want to compare only date part of DateTime, not time part:
DateTime d1 = DateTime.Parse("10/10/2011");
DateTime d2 = DateTime.Parse("01/01/2011");
if (d1.Date > d2.Date)
{
// do the stuff
}

DateTime.Now.ToString("MM/dd/yyyy");
DateTime.Today.ToString("MM/dd/yyyy");

DateTime.Parse is what you are looking for...

convert string to date without time

This line
System.DateTime.Parse("09/12/2009");
convert date (string) to 9/12/2009 12:00:00 AM. How can I get a date in the form 9/12/2009.
after explanations I do:
DateTime dt = System.DateTime.Parse(Request.Form["datepicker"]);
dt.ToString("dd/mm/yyyy");
/* and this code have time, why???*/

Your problem is not with the parsing but with the output. Look at how ToString works for DateTime, or use this example:
using System;
class Program
{
static void Main(string[] args)
{
DateTime dt = DateTime.Parse("09/12/2009");
Console.WriteLine(dt.ToString("dd/MM/yyyy"));
}
}
Or to get something in your locale:
Console.WriteLine(dt.ToShortDateString());
Update: Your update to the question implies that you do not understand fully my answer yet, so I'll add a little more explanation. There is no Date in .NET - there is only DateTime. If you want to represent a date in .NET, you do so by storing the time midnight at the start of that day. The time must always be stored, even if you don't need it. You cannot remove it. The important point is that when you display this DateTime to a user, you only show them the Date part.

Anything parsed to a DateTime object will contain date+time. If no time is sepcified, the assumed value will be 0000hr.
To get the date-only representation, it's up to how you format it to string.
E.g. theDate.ToString("dd/MM/yyyy")
Refer to the MSDN Date and Time Format Strings.

if you want to convert gridview datetime column to date only column use this code :
raddgvDaybook.Rows.ToList().ForEach(x =>
{
DateTime dt = DateTime.Parse(x.Cells["Vocdate"].Value.ToString());
string str = string.Format("{0:MM/dd/yyyy}", dt);
x.Cells["Vocdate"].Value = str;
});
I tested the code it will work if you bind the datetime as string in dgv.