Develop Reference

Add the seconds to date to calculate Time

Quick question, I've got this Program which gives me the seconds past since 01.01.2000.
E.g. the Seconds for 12.02.2016 11:23:55 would roughly be 508590531 seconds.
Is there a Method that given the seconds past and the start date, that calculates the end date?
I am aware that I could calculate the Minutes/Days/Months/Years that passed by hand, and then try to calculate the 'endTime' but I'd have to take care that I don't make a mistake by missing out a leap year for instance.
Quick Definition just to be totally clear:
Start Date : 01.01.2000
End Date: Current Date
Time passed: Seconds passed between start date and end date

It's easy. Just add the seconds, no magic or new solution required.
DateTime AddedFrom2000 = new DateTime(2000, 1, 1).AddSeconds(508590531);

Sure.
You can subtract to each other and use TimeSpan.TotalSeconds property like;
var ts = DateTime.Now - new DateTime(2000, 1, 1);
Console.WriteLine(ts.TotalSeconds);
If you wanna get it as an int, you can explicitly cast it as (int)ts.TotalSeconds.
If you already have seconds and you want to calculate based on your start date, you can use AddSeconds method like;
var dt = new DateTime(2000, 1, 1).AddSeconds(508590531); // {12.02.2016 11:08:51}

var newDate = oldDate.AddSeconds(numSeconds);

TotalHours function error handling

I have two textboxes for each day of the week which allow a user to enter a start time and an end time in this format (hh:mm) (24 hour clock). For example, if a user enters 1:35 in the first textbox for Monday and 3:30 in the second textbox for Monday and press the 'Calculate' button, it will return the decimal 1.92. It runs and returns the result to a label, but I'm having some problems with my error handling.
If a textbox is left empty it will throw an error when ran and display "String was not recognized as a valid TimeSpan." How can I fix this so if a user doesn't enter a time it will either assume '0' or not run the code with that particular set of empty textboxes?
//Monday
TimeSpan Mon1In = TimeSpan.Parse(TextBoxInMon1.Text);
TimeSpan Mon1Out = TimeSpan.Parse(TextBoxOutMon1.Text);
MonLabel1.Text = (Mon1Out - Mon1In).TotalHours.ToString("f2");
//Tuesday
TimeSpan Tues1In = TimeSpan.Parse(TextBoxInTues1.Text);
TimeSpan Tues1Out = TimeSpan.Parse(TextBoxOutTues1.Text);
TuesLabel1.Text = (Tues1Out - Tues1In).TotalHours.ToString("f2");
//Wednesday
TimeSpan Wed1In = TimeSpan.Parse(TextBoxInWed1.Text);
TimeSpan Wed1Out = TimeSpan.Parse(TextBoxOutWed1.Text);
WedLabel1.Text = (Wed1Out - Wed1In).TotalHours.ToString("f2");
all the way to Friday.

You should use TimeSpan.TryParseMSDN where the out param is the value you would like and if the tryparse fails you set the TimeSpan to 00:00:00.
//Monday
TimeSpan Mon1In,Mon1Out;
if(!TimeSpan.TryParse(TextBoxInMon1.Text,out Mon1In)) Mon1In = default(TimeSpan);
if(!TimeSpan.TryParse(TextBoxOutMon1.Text,out Mon1Out)) Mon1Out = default(TimeSpan);
MonLabel1.Text = (Mon1Out - Mon1In).TotalHours.ToString("f2");

Difference between two datetimes

I have a datetime, I want to show the difference from DateTime.Now to received datetime and bind it. The result should be something like this:
1d 15h 13m 7s
What is the best way to do it? StringFormat? IValueConverter?

I'd suggest using the Timespans ToString method and custom TimeSpan format strings
Timespans if you aren't already aware are designed for measuring time intervals like this and can be convenienty obtained by subtracting one date from another.
var startDate = new DateTime(2013,1,21);
var currentDate = DateTime.Now;
TimeSpan interval = currentDate - startDate;
string intervalInWords = String.Format("{0:%d} days {0:%h} hours {0:%m} minutes {0:%s} seconds", interval);
Console.WriteLine(intervalInWords);
This will print out something like
267 days 10 hours 45 minutes 21 seconds
As has been noted in comments because these datetimes may be in different timezones/daylight saving times you should be very careful using this technique. Using UTCtime for both which is consistent throughout the whole year should be sufficient if that is feasible. In general it is often best policy to save all datetimes as UTC along with the timezone/offset (if required) and then if they are needed in a specific timezone offset convert on display.

Use TimeSpan
Example:
DateTime oldDate = new DateTime(2002,7,15);
DateTime newDate = DateTime.Now;
// Difference in days, hours, and minutes.
TimeSpan ts = newDate - oldDate;
// Difference in days.
int differenceInDays = ts.Days;
Now you can change it according to your requirement.

The other answers are correct from the formatting point of view, but just to address the WPF angle, I'm guessing you want to update a label/textbox so it constantly contains an accurate duration?
If so, you can do this with the timer and the dispatcher.
Timer code:
//duration in milliseconds, 1000 is 1 second
var timer = new Timer(1000);
timer.Elapsed += timer_Elapsed;
timer.Start();
Timer elapsed code:
//this is set elsewhere
private readonly DateTime _received;
void timer_Elapsed(object sender, ElapsedEventArgs e)
{
Application.Current.Dispatcher.Invoke(
DispatcherPriority.Normal,
new Action(()
=> //replace label1 with the name of the control you wish to update
label1.Content =
string.Format("{0:%d} days {0:%h} hours {0:%m} minutes {0:%s} seconds"
, (DateTime.Now - _received))));
}

You can use TimeSpan, also look out [here][1]
[1]: Showing Difference between two datetime values in hours i would suggest you to go through TimeSpan.
DateTime startDate = Convert.ToDateTime(2008,8,2);
DateTime endDate = Convert.ToDateTime(2008,8,3);
TimeSpan duration = startDate - endDate;

Create a property like DateProp of type DateTime to which you'll bind on your XAML , and assuming your property is Other_date_here, initialize it like this:
DateProp = DateTime.Now.Subtract(Other_date_here);
Last, on your XAML, bind it and set the formatting like this:
Text="{Binding Date, StringFormat=d day H hours m minutes s seconds}"
(or whatever other format you like:).

Display duration in milliseconds

I want to display duration with milliseconds on a web page. So far I have done this:
I managed to display this output on a label: 00:02:50, but I want to display milliseconds as well, so the result should look like this 00:02:50:000. How do I achieve this?
Code behind:
protected void Page_Load(object sender, EventArgs e)
{
DateTime startTime = DateTime.Now;
// sleep for 2.5s
Thread.Sleep(2500);
DateTime stopTime = DateTime.Now;
TimeSpan duration = stopTime - startTime;
Result.Text = duration.ToString("mm':'ss':'ff");
}

First of all, if you're timing things I would recommend using the StopWatch class as that's what it's there for. You can find it in the System.Diagnostics namespace: System.Diagnostics.Stopwatch.
You can instantiate a new one and start measuring the elapsed amount of time with one line of code: var stop = System.Diagnostics.Stopwatch.StartNew(); and then stop the timer with the stop method: stop.Stop();. You can then return the elapsed time using the Elapsed property var elapsed = stop.Elapsed;.
Then in order to display the elapsed time with milliseconds you would call the ToString method on the elapsed timespan with the correct parameters.
So putting it all together your code would look like this:
protected void Page_Load(object sender, EventArgs e)
{
var timer = System.Diagnostics.Stopwatch.StartNew();
// sleep for 2.5s
Thread.Sleep(2500);
timer.Stop();
var elapsed = timer.Elapsed;
Result.Text = elapsed.ToString("mm':'ss':'fff");
}
Hope that helps!
James

Your current code should be displaying minutes, seconds, and hundredths of a second.
Result.Text = duration.ToString("mm':'ss':'ff");
To display milliseonds instead of hundredths of a second:
// output: 00:02:500
Result.Text = duration.ToString("mm':'ss':'fff");
See the documentation for Custom Date and Time Format Strings.

The doc says: "fff" gives you:
The milliseconds in a date and time value.
You're using "ff" which gives you:
The hundredths of a second in a date and time value.
So, change your code to:
duration.ToString("mm':'ss':'fff");

I think you're confused. In your case 00:02:50 means 2 seconds and 50 hundredths of second. If you want to display milliseconds, use format like mm':'ss':'fff (notice the one added f). This will print something like 00:02:500, i.e. 2 seconds and 500 thousandths of second, or 2 s 500 ms.
But this doesn't mean your measurements will be precise down to millisecond. That's not what DateTime.Now is meant to do. If you want to make measurements this precise, you should use StopWatch.

Use TimeSpan.ToString Method with custom format.
The returned string is formatted with the "c" format specifier and has the following format:
[-][d.]hh:mm:ss[.fffffff]
Elements in square brackets ([ and ]) may not be included in the returned string. Colons and periods (: and.) are literal characters.
Result.Text = duration.ToString("mm:ss:fff");
or
Result.Text = duration.ToString("hh:mm:ss.fff");
Ref :Custom Date and Time Format Strings, The "fff" Custom Format Specifier
DateTime date1 = new DateTime(2008, 8, 29, 19, 27, 15, 18);
CultureInfo ci = CultureInfo.InvariantCulture;
Console.WriteLine(date1.ToString("hh:mm:ss.fff", ci));
// Displays 07:27:15.018

Otherwise just use the properties from the timespan like this:
var result = String.Format("{0}:{1}:{2}", duration.Minutes, duration.Seconds, duration.Milliseconds);
Result.Text = result
This way I think you gain more control over what you want to display, instead of formatting the timespan in the ToString()-method which more easily allows typos to be made...
Hope this helps!
Update:
To add the hours as well this is how it'll look like:
var result = String.Format("{0}:{1}:{2}:{3}", duration.Hours, duration.Minutes, duration.Seconds, duration.Milliseconds);

Differences between 2 times?

StartCopying = DateTime.Now;
File.Copy(#"C:\Users\sshap5x\Desktop\line4\shiraDebbi\Hackers.avi", #"C:\Users\sshap5x\Desktop\test\Hackers.avi", true);
EndCopying = DateTime.Now;
CopyingTime1 = (endCopying - startCopying).Duration;
What is the problem with my code?
Copying time is TimeSpan object.

The assignments are to StartCopying and EndCopying but your read the data from other variables startCopying and endCopying.
C# is case sensitive.
And also Duration is a method. so you need to use .Duration()
And as suggested in the comments to your question, for better resolution use the Stopwatch class.

// This prints 0.1 (roughly)
DateTime start = DateTime.Now;
Thread.Sleep(100);
var diff = DateTime.Now - start;
Console.WriteLine(diff.TotalSeconds);
In your code (which wouldn't even compile) you use .Duration as a property when it is in fact a method. In any case, you don't need to call Duration at all because the result returned by the subtraction is a TimeSpan containing the difference. You can then get this diff in whatever format you need (milliseconds, seconds, hours, days, etc.).
The only need for the Duration method is if you are unsure whether the result is negative or positive.

Did you forget the () for Duration and capitalisation for the variables EndCopying and StartCopying?
DateTime StartCopying = DateTime.Now;
DateTime EndCopying = DateTime.Now;
TimeSpan CopyingTime1 = (EndCopying - StartCopying).Duration();