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How to remove hour, minutes and seconds from a date

I've two types of dates, one in DateTime format and another in string format, both dates having the following format:
yyyy-MM-dd HH: mm: ss
I want to delete HH: mm: ss because I need to compare these dates in a loop to iterate through a database. The problem's that one of these dates is returned by a CalendarSelectionDate event, and the hour, minutes and seconds are even set to 0. Anyone have the best way to do this?
UPDATE:
if (DateTime.TryParseExact(reader["data"].ToString(), "yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture,
DateTimeStyles.None, out dt)){...}
The code behavior return an invalid date, in particular if I've 12/05/15 ... the code will return 1/01/0001

If you want to compare DateTime objects without the hour, you can use the Date property:
if (myDbDate.Date != myUserDate.Date) { }
You can also cast the date to a string using ToString(), but be aware that dates are a notoriously very hard thing to deal with when they are strings:
if (myDbDate.ToShortDateString() != myUserDate) { }
or if you are very sure of your format, you can use a custom date format:
if (myDbDate.ToString("yyyy-MM-dd") != myUserDate) { }
Update
Automatically parsing the string to a date (with DateTime.Parse or TryParse) has often resulted, in my own and personal experience, in very random results. You never seem to know which format .Net will decide on using (dd/MM or MM/dd ?).
Using ParseExact or TryParseExact solves this problem, and allows to work on the date further (add days, for instance). But for a simple comparison as in the initial question, since you're "locking" the date format in the code, it doesn't change much (maybe performance-wise, I don't know), and it's much more simple to cast the date to a string than the other way.
That being said, I went on the assumption that the comparison was "is different". If the comparison is "is later/earlier than", casting to a date would indeed be the right solution.

First you have to understand that DateTime does not have a format. It only contains information that describes a specific point in time. Formats apply to the string representations of a DateTime. For what you want you can use DateTime.Date which will return a new DateTime with the same year, month, and day values, but with the time set to 12 AM. That along with DateTime.ParseExact will allow you to parse the string to a DateTime then compare just the Date part.
var someDate = DateTime.ParseExact(stringValue, "yyyy-MM-dd HH: mm: ss");
if(someDate.Date != otherDate.Date)
{
}

To get the base date of any DateTime, simply use the Date property.
DateTime.Now.Date

A Datetime without time without string.format

I have a DataTable coming from a stored procedure which I'm writing to an excel file. There's a column with a DateTime datatype, and looking at the values in there, they're just generic dates with the time stamp.
I've tried using the DateTime.Date property, but that still gives me a time stamp. Further, I've tried to create a new DateTime object using the year,month,day constructor but it still adds a time stamp:
DateTime newDate = DateTime(oldDate.Year, oldDate.Month, oldDate.Day);
I'm trying to keep the column datatype to DateTime but remove the time stamp, so this rules out ToString("") formatting. Is there another way?

A DateTime always contains a date and a time portion. If you use the Date property it returns a new DateTime where the time is 0:00:00 so midnight at the same day. You want a string representation of the datetime without the time.
You can use DateTime.ToString:
string result = oldDate.ToString("d"); // uses current culture's date format
or
string result = oldDate.ToShortDateString(); // same as above
or
string result = oldDate.ToString("MM-dd-yyyy"); // custom format
Edit: "so this rules out ToString("") formatting. Is there another way?"
No, because of the reason mentioned above.
It's important to separate the data from how it is displayed. If you need to display it without time use the code above, you can store the original DateTime variable for future processing, select it again from database or use DateTime.Parse/DateTime.ParseExcact to get a DateTime from the string.

The 'problem' is that there is no Date struct in .NET, you only have a DateTime struct. That will always contain both date and time. You can only format it as date.
Or, you could of course write your own struct containing only the date part, or give up and use string.Format to format it as a date (possibly using the short date string d).

If you mean time part with timestamp, a DateTime instance always have both date and time part. DateTime.Date property just sets the time value set to midnight.
You can get it's string representation if you want only it's Date part. You can get standard date and time format or custom date and time format with DateTime.ToString() method.
Usually, you can use ShortDatePattern to get only string representation of Date part which uses standard "d" format of your CurrentCulture.
DateTime.Now.ToString("d");
There is a proposal for System.Date and System.Time types for .NET Framework in dotnet/corefx on GitHub page by the way.
Proposal: System.Date type

You should use DateTime.Now.ToShortDateString();

Formatting DateTime value in C#

I have a datetime variable like this:
DateTime date1 = DateTime.Now; // has 9.4.2014 01:12:35
I want to assign this to another datetime or change its value like this:
2014-04-09 13:12:35
How can I do?
Thanks.
EDIT : I don't want string variable. I want it Datetime format.

try this :
date1.ToString("yyyy-MM-dd HH:mm:ss")
Also look at the table below here http://msdn.microsoft.com/en-us/library/8kb3ddd4(v=vs.110).aspx
edit :
As Jon said (and which I didn't mention) :
you should add InvariantCulture ( if you dont want it to be used with current thread culture ) :
CultureInfo heIL = new CultureInfo("he-IL");
heIL.DateTimeFormat.Calendar = new HebrewCalendar();
CultureInfo dft = Thread.CurrentThread.CurrentCulture;
Thread.CurrentThread.CurrentCulture = heIL;
Check these :
DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss")
DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss",CultureInfo.InvariantCulture);
result ( I live in israel) :
תשע"ד-ח'-ט' 13:32:31
2014-04-09 13:32:31

The code you've written just assigns a value to a variable. It doesn't return anything, and it doesn't have any inherent string representation. A DateTime value is just a date/time. It can be displayed in whatever format you want, but that's not part of the value of the variable.
It sounds like you're wanting to convert it to a string in a particular format, which you should do with DateTime.ToString - but only when you really need to. Try to keep the value as a DateTime for as long as possible. Typically you only need to convert to a string in order to display the value to a user, or possibly to use it in something like JSON. (If you find yourself converting it to a string for database usage, you're doing it wrong - make sure your schema has an appropriate data type for the field, use a parameterized query, and set the parameter value to just the DateTime - nor formatting required.)
The format you've specified looks like it's meant to be a machine-readable one rather than a culture-specific one, so I'd suggest:
string text = date1.ToString("yyyy-MM-dd HH:mm:ss",
CultureInfo.InvariantCulture);
By specifying the invariant culture, we've said that the result shouldn't depend on the current culture (which otherwise it would) - this can make a big difference if the current culture uses a different calendar system, for example.

Something like the following, which is one of the constructors of the DateTime object:
d = new DateTime(2014, 5, 6, 5, 4, 30);
Which will set d to 06/05/2014 05:04:30. Its parameters are in descending size order, so Year, Month, Day, Hour, Minute then Seconds.
If you want to adjust the time by an amount, look at the add methods, or TimeSpans.

you can just use something like this to format the date:
date1.ToString("dd/MM/yyyy HH:mm:ss")
By using "HH" instead of "hh" you will get 24hour format on the hour.
Hope that helps.

Try this
DateTime date1 = DateTime.Now;
string datestring=date1.ToString("yyyy-MM-dd HH:mm:ss",CultureInfo.InvariantCulture)
http://msdn.microsoft.com/en-us/library/8kb3ddd4(v=vs.110).aspx

As a guess, the "returned" value of your DateTime object is seen by you, by hoovering over the variable in the IDE while debugging.
This is just another form of calling internally the default ToString() method of your DateTime object by the debugger. The value is the same.
See: system.datetime
DateTime Values and their string representations
Internally, all DateTime values are represented as the number of ticks (the number of 100-nanosecond intervals) that have elapsed since 12:00:00 midnight, January 1, 0001. The actual DateTime value is independent of the way in which that value appears when displayed in a user interface element or when written to a file. The appearance of a DateTime value is the result of a formatting operation. Formatting is the process of converting a value to its string representation.
Because the appearance of date and time values is dependent on such factors as culture, international standards, application requirements, and personal preference, the DateTime structure offers a great deal of flexibility in formatting date and time values through the overloads of its ToString method. The default DateTime.ToString() method returns the string representation of a date and time value using the current culture's short date and long time pattern.

date month not displaying correctly

why is
string date = string.Format("{0:mmddyyHHmmss}",
DateTime.Now);
giving me 420813104204
shouldn't it be 040813... ?
I am trying to match
mmddyyHHmmSS

You need to use MM for month not mm, mm is for minutes not months.
string date = string.Format("{0:MMddyyHHmmss}",
You can find more about formats here.

try this instead
string date = string.Format("{0:MMddyyHHmmss}", DateTime.Now);

string date = string.Format("{0:MMddyyHHmmss}", DateTime.Now);
would give you the required format.
Check out this link for reference

That's because mm is for the minute not months, you need to use MM. You can see the definition for custom date time strings here.
string date = string.Format("{0:MMddyyHHmmss}",
Further, I generally don't use string.Format with DateTime objects because I've seen some anomalies when it comes to parsing them across different cultures. Leveraging the ToString method on the DateTime object for me has always been more reliable. That's just something I've seen - it also could be that I was doing something wrong with the string.Format. I wish I could build an example of that right now but I don't even remember what those anomalies are now - I just remember having problems so I switched.

Storing a short date in a DateTime object

I'm trying to store a shortened date (mm/dd/yyyy) into a DateTime object. The following code below is what I am currently trying to do; this includes the time (12:00:00 AM) which I do not want :(
DateTime goodDateHolder = Convert.ToDateTime(DateTime.Now.ToShortDateString());
Result will be 10/19/2009 12:00:00 AM

DateTime is an integer interpreted to represent both parts of DateTime (ie: date and time). You will always have both date and time in DateTime. Sorry, there's nothing you can do about it.
You can use .Date to get the date part. In these cases, the time will always be 12:00 but you can just ignore that part if you don't want it.

You only have two options in this situation.
1) Ignore the time part of the value.
2) Create a wrapper class.
Personally, I am inclined to use option 1.

A DateTime will always have a time component - even if it is 12:00:00 AM. You just need to format the DateTime when you display it (e.g. goodDateHolder.ToShortDateString()).

Instead of .Now you can use .Today which will not remove the time part, but will only fill the date part and leave time to the default value.
Later on, as others pointed out, you should try to get the date part ignoring the time part, depending on the situation.

You'll always get the time portion in a DateTime type.
DateTime goodDateHolder = Convert.ToDateTime(DateTime.Now.ToShortDateString());
will give you today's date but will always show the time to be midnight.
If you're worried about formatting then you would try something like this
goodDateHolder.ToString("mm/dd/yyyy")
to get the date in the format that you want.
This is a good resource msdn-dateformat

You can also check out Noda Time based off the Java Joda Time library.

DateTime object stores both the date and the time. To display only the date, you would use the DateTime.ToString(string) method.
DateTime goodDateHolder = DateTime.Now;
// outputs 10/19/2009
Console.WriteLine(goodDateHolder.ToString("MM/dd/yyyy"));
For more information on the ToString method, follow this link

You might not be able to get it as a DateTime object...but when you want to display it you can format it in the way you want by doing something like.
myDateTime.ToString("M/d/yyyy") which gives 10/19/2009 for your example.

DateTime is merely a UInt64 with useful and clever formatting wrapped around it to make it appear like a date plus a time. You cannot eliminate the time element.