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offset datetime with each other - Must tell the number of days in total

I must have built up such that I have a datetime which gets added antale day as it should go forward. and then I have time as it should set off in relation to 04/10/16 to 10/09/16
I do not care about the time which is in datetime. It should not I use for anything. What I need out of this is exactly how many days there are from that time.
Datetime dateString = "4/10/2016 8:30:52" //I pretend that it comes from the database, it was more in terms of see what come there.
DateTime dt = DateTime.Now.AddDays(5);
What I need out of this is that it tell me how many days there are in between the two datetime as I entered.

You can substract DateTime objects to obtain a TimeSpan:
Datetime dateString = DateTime.Parse("4/10/2016 8:30:52");
DateTime dt = DateTime.Now;
TimeSpan duration = dt-dateString;
From the TimeSpan object, you can get how many (full) days with :
int totalCompleteDays = (int)duration.TotalDays;
Or if you want a rounded results :
int roundedTotalDays = (int)Math.Round(duration.TotalDays);

DateTime objects support basic operators and will return TimeSpan objects.
DateTime DateTimeB = DateTime.Now.AddDays(5);
DateTime DateTimeA = DateTime.Now;
TimeSpan difference = DateTimeA - DateTimeB;
...
you can then use the TotalDays property of the timeSpan.
...
Console.out.WriteLine(difference.TotalDays);

Subtracting two dates

I have two calendars and each return a DateTime from calendar.SelectedDate.
How do I go about subtracting the two selected dates from each other, giving me the amount of days between the two selections?
There is a calendar.Subtract() but it needs a TimeSpan instead of DateTime.

You can use someDateTime.Subtract(otherDateTime), this returns a TimeSpan which has a TotalDays property.

Just use:
TimeSpan difference = end - start;
double days = difference.TotalDays;
Note that if you want to treat them as dates you should probably use
TimeSpan difference = end.Date - start.Date;
int days = (int) difference.TotalDays;
That way you won't get different results depending on the times.
(You can use the Subtract method instead of the - operator if you want, but personally I find it clearer to use the operator.)

Think about it.
How do you express a difference betwen two dates? With another date?
That's why you need the TimeSpan
DateTime dtToday = new System.DateTime(2012, 6, 2, 0, 0, 0);
DateTime dtMonthBefore = new System.DateTime(2012, 5, 2, 0, 0, 0);
TimeSpan diffResult = dtToday.Subtract(dtMonthBefore);
Console.WriteLine(diffResult.TotalDays);

TimeSpan to DateTime conversion

I want to convert a Timespan to Datetime. How can I do this?
I found one method on Google:
DateTime dt;
TimeSpan ts="XXX";
//We can covnert 'ts' to 'dt' like this:
dt= Convert.ToDateTime(ts.ToString());
Is there any other way to do this?

It is not very logical to convert TimeSpan to DateTime. Try to understand what leppie said above. TimeSpan is a duration say 6 Days 5 Hours 40 minutes. It is not a Date. If I say 6 Days; Can you deduce a Date from it? The answer is NO unless you have a REFERENCE Date.
So if you want to convert TimeSpan to DateTime you need a reference date. 6 Days & 5 Hours from when? So you can write something like this:
DateTime dt = new DateTime(2012, 01, 01);
TimeSpan ts = new TimeSpan(1, 0, 0, 0, 0);
dt = dt + ts;

While the selected answer is strictly correct, I believe I understand what the OP is trying to get at here as I had a similar issue.
I had a TimeSpan which I wished to display in a grid control (as just hh:mm) but the grid didn't appear to understand TimeSpan, only DateTime . The OP has a similar scenario where only the TimeSpan is the relevant part but didn't consider the necessity of adding the DateTime reference point.
So, as indicated above, I simply added DateTime.MinValue (though any date will do) which is subsequently ignored by the grid when it renders the timespan as a time portion of the resulting date.

TimeSpan can be added to a fresh DateTime to achieve this.
TimeSpan ts="XXX";
DateTime dt = new DateTime() + ts;
But as mentioned before, it is not strictly logical without a valid start date. I have encountered
a use-case where i required only the time aspect. will work fine as long as the logic is correct.

You need a reference date for this to be useful.
An example from
http://msdn.microsoft.com/en-us/library/system.datetime.add.aspx
// Calculate what day of the week is 36 days from this instant.
System.DateTime today = System.DateTime.Now;
System.TimeSpan duration = new System.TimeSpan(36, 0, 0, 0);
System.DateTime answer = today.Add(duration);
System.Console.WriteLine("{0:dddd}", answer);

Worked for me.
var StartTime = new DateTime(item.StartTime.Ticks);

If you only need to show time value in a datagrid or label similar, best way is convert directly time in datetime datatype.
SELECT CONVERT(datetime,myTimeField) as myTimeField FROM Table1

You could also use DateTime.FromFileTime(finishTime) where finishTme is a long containing the ticks of a time. Or FromFileTimeUtc.

An easy method, use ticks:
new DateTime((DateTime.Now - DateTime.Now.AddHours(-1.55)).Ticks).ToString("HH:mm:ss:fff")
This function will give you a date (Without Day / Month / Year)

A problem with all of the above is that the conversion returns the incorrect number of days as specified in the TimeSpan.
Using the above, the below returns 3 and not 2.
Ideas on how to preserve the 2 days in the TimeSpan arguments and return them as the DateTime day?
public void should_return_totaldays()
{
_ts = new TimeSpan(2, 1, 30, 10);
var format = "dd";
var returnedVal = _ts.ToString(format);
Assert.That(returnedVal, Is.EqualTo("2")); //returns 3 not 2
}

First, convert the timespan to a string, then to DateTime, then back to a string:
Convert.ToDateTime(timespan.SelectedTime.ToString()).ToShortTimeString();

Exact c# result of sql datediff

I'm trying to get the number of days (calculated byu datediff) in sql and the number of days in c# (calculated by DateTime.now.Substract) to be the same, but they return different results....
//returns 0
int reso = DateTime.Now.Subtract(expirationDate).Days;
vs
//returns 1
dateDiff(dd,getDate(),ExpirationDate)
In both cases, ExpirationDate is '10/1/2011 00:00:00', and the code and the DB are sitting on the same server. I want the return int to be the same. I suspect I'm missing something stupid... ideas??

dateDiff(dd,getDate(),ExpirationDate) Is doing a days comparison. DateTime.Now.Subtract(expirationDate).Days is doing a date and time
For example
SELECT dateDiff(dd,'10/1/2011 23:59:00' , '10/2/2011') returns one day even when only one minute apart.
If you want the same in C# you need to remove the time component
e.g.
DateTime dt1 = new DateTime(2011,10,1, 23,59,0);
DateTime dt2 = new DateTime(2011,10,2, 0,0,0);
Console.WriteLine((int) dt2.Subtract(dt1.Subtract(dt1.TimeOfDay)));
So in your case it would be something like
DateTime CurrentDate = DateTime.Now;
int reso = CurrentDate.Subtract(CurrentDate.TimeOfDay).Subtract(DateTime.expirationDate).Days;
I haven't tested it but I would not do
DateTime.Now.Subtract(DateTime.Now.Subtract.TimeOfDay)
Because the second call to Now wouldn't be guaranteeing to be the same as first call to Now
In any case Stealth Rabbi's answer seems more elegant anyway since you're looking for a TimeSpan not a DateTime

10/1/2011 is less than 1 day away from DateTime.Now. Since you're getting back a TimeSpan and then applying Days to it, you're getting back a TimeSpan that is < 1 day. So it'll return 0 Days.
Instead, just use the Date component of those DateTimes and it'll correctly report the number of days apart - like this:
DateTime now = DateTime.Now;
DateTime tomorrow = new DateTime(2011, 10, 1);
var val = (tomorrow.Date - now.Date).Days;
This will yield you 1 day.

I'm assuming you want the number of Total days, not the number of days from the largest previous unit. You'd want to use the TotalDays property. Also, you may find it easier to use the minus operator to do a subtraction
DateTime d1 = DateTime.Now;
DateTime d2 = new DateTime(2009, 1, 2);
TimeSpan difference = d1 - d2;
Console.WriteLine(difference.TotalDays); // Outputs (today):1001.46817997424

What is the easiest way to subtract time in C#?

I'm trying to put together a tool that will help me make work schedules. What is the easiest way to solve the following?
8:00am + 5 hours = 1:00pm
5:00pm - 2 hours = 3:00pm
5:30pm - :45 = 4:45
and so on.

These can all be done with DateTime.Add(TimeSpan) since it supports positive and negative timespans.
DateTime original = new DateTime(year, month, day, 8, 0, 0);
DateTime updated = original.Add(new TimeSpan(5,0,0));
DateTime original = new DateTime(year, month, day, 17, 0, 0);
DateTime updated = original.Add(new TimeSpan(-2,0,0));
DateTime original = new DateTime(year, month, day, 17, 30, 0);
DateTime updated = original.Add(new TimeSpan(0,-45,0));
Or you can also use the DateTime.Subtract(TimeSpan) method analogously.

Check out all the DateTime methods here: http://msdn.microsoft.com/en-us/library/system.datetime.aspx
Add Returns a new DateTime that adds the value of the specified TimeSpan to the value of this instance.
AddDays Returns a new DateTime that adds the specified number of days to the value of this instance.
AddHours Returns a new DateTime that adds the specified number of hours to the value of this instance.
AddMilliseconds Returns a new DateTime that adds the specified number of milliseconds to the value of this instance.
AddMinutes Returns a new DateTime that adds the specified number of minutes to the value of this instance.
AddMonths Returns a new DateTime that adds the specified number of months to the value of this instance.
AddSeconds Returns a new DateTime that adds the specified number of seconds to the value of this instance.
AddTicks Returns a new DateTime that adds the specified number of ticks to the value of this instance.
AddYears Returns a new DateTime that adds the specified number of years to the value of this instance.

This works too:
System.DateTime dTime = DateTime.Now();
// tSpan is 0 days, 1 hours, 30 minutes and 0 second.
System.TimeSpan tSpan = new System.TimeSpan(0, 1, 3, 0);
System.DateTime result = dTime + tSpan;
To subtract a year:
DateTime DateEnd = DateTime.Now;
DateTime DateStart = DateEnd - new TimeSpan(365, 0, 0, 0);

Hi if you are going to subtract only Integer value from DateTime then you have to write code like this
DateTime.Now.AddHours(-2)
Here I am subtracting 2 hours from the current date and time

Use the TimeSpan object to capture your initial time element and use the methods such as AddHours or AddMinutes. To substract 3 hours, you will do AddHours(-3). To substract 45 mins, you will do AddMinutes(-45)

try this
namespace dateandtime
{
class DatesTime
{
public static DateTime Substract(DateTime now, int hours,int minutes,int seconds)
{
TimeSpan T1 = new TimeSpan(hours, minutes, seconds);
return now.Subtract(T1);
}
static void Main(string[] args)
{
Console.WriteLine(Substract(DateTime.Now, 36, 0, 0).ToString());
}
}
}

TimeLeftToOpen= new TimeSpan(TimeLeftToOpen.Hours, TimeLeftToOpen.Minutes, TimeLeftToOpen.Seconds - 1);