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TimeSpan calculation is different between TimeOnly and DateTime

I'm having a hard time understanding why this is happening or even if this should be happening. If I calculate the TimeSpan between two DateTime objects (same date, different times) and compare it to the same calculation using two TimeOnly objects I get different results.
var start = new DateTime(1, 1, 1, 14, 0, 0);
var end = new DateTime(1, 1, 1, 10, 0, 0);
Console.WriteLine(end - start); // Prints -4 hours
However...
var start = new TimeOnly(14, 0, 0);
var end = new TimeOnly(10, 0, 0);
Console.WriteLine(end - start); // Prints 20 hours???
Isn't the span between starting at 2pm and ending at 10am always a span of -4 hours? Interestingly enough if I take the second one and do Console.WriteLine(end.ToTimeSpan() - start.ToTimeSpan()); I end up with -4 hours.
This feels like an error on TimeOnly's part but I don't know. Here is a fiddle I did comparing results between NodaTime, System.DateTime, converting System.TimeOnly to TimeSpan, and System.TimeOnly.

Convert 12-hour time format to 24 hour integer?

In my app I have a drop-down box of strings that shows possible hours in 12-hour time for the user to select. The possible values are:
9am
10am
11am
12pm
1pm
2pm
3pm
4pm
5pm
What code will convert one of these strings to a 24 hour integer? For example, 10am should be converted to 10 and 4pm should be converted to 16.

You can use DateTime.Parse(...) to get a DateTime value, and then reference the .Hour property for a result;
int h = DateTime.Parse("10am").Hour; // == 10
int h2 = DateTime.Parse("10pm").Hour; // == 22
DateTime.Parse is pretty liberal in what it allows, but obviously makes some assumptions internally. For example, in the above code, DateTime.Parse("10am") returns 10am on the current date in the current timezone (I think...). So, be aware of the context in which you use the API.

If you have a dropdown, why not set the values to be the integer values you desire:
<asp:DropDownList runat="server" ID="hours">
<asp:ListItem Value="9">9am</asp:ListItem>
<asp:ListItem Value="10">10am</asp:ListItem>
<!-- etc. -->
<asp:ListItem Value="17">5pm</asp:ListItem>
</asp:DropDownList>

Considering the times are continuous, you can simplify the logic:
var firstHourStr = box.Items[0].ToString();
var firstHour = int.Parse(firstHourStr.Replace("am", "").Replace("pm", ""));
if (firstHourStr.Contains("pm"))
{
firstHour += 12;
}
var selectedHour = firstHour + box.SelectedIndex;
If the hours are static, and you know the first hour, you could have a const and simplify the process by much with var selectedHour = FIRST_HOUR + box.SelectedIndex.
Also, I assumed valid formats as shown in the question.
Final note: You'll need to handle the 12pm case which causes problems, due to the nature of the hour 12 being a single second after "am".

You could use DateTime.Parse, but that would not play nicely with internationalization.
int hour = DateTime.Parse(stringValue).Hour;
Instead, just use DateTime objects in the ComboBox and format them using FormatString:
// In Constructor:
cbHours.Items.Add(new DateTime(2000, 1, 1, 8, 0, 0));
cbHours.Items.Add(new DateTime(2000, 1, 1, 10, 0, 0));
cbHours.Items.Add(new DateTime(2000, 1, 1, 13, 0, 0));
cbHours.FormatString = "h tt";
// In event handler
if (cbHours.SelectedIndex >= 0)
{
int hour = ((DateTime)cbHours.SelectedItem).Hour
// do things with the hour
}

how to subtract previous date from current date using c# [duplicate]

This question already has answers here:
C#: how do I subtract two dates?
(11 answers)
Closed 10 years ago.
i want to subtract previous date from current date. previous date may be 2 months and 15 days or 1 year 9 month and 10 days... like this etc... So please how can i write the Coding in C#. thanks a lot.

Any answer using TimeSpan will not be able to give you "2 months and 15 days" - as the length of a month changes over time.
Doing this using the base class libraries is a pain... which is one of the reasons I started the Noda Time project. Amongst its other features, it allows you to determine the Period between to dates (or dates and times, etc).
For example, let's see how long I've been on Stack Overflow:
LocalDate today = new LocalDate(2013, 2, 8);
LocalDate start = new LocalDate(2008, 9, 26);
// This defaults to using year, month, day units. You can specify alternatives.
Period period = Period.Between(start, today);
Console.WriteLine("{0} years; {1} months; {2} days",
period.Years, period.Months, period.Days);
Output:
4 years; 4 months; 13 days
Or if you actually wanted to subtract a period from a date (the question isn't very clear) you can do that too:
Period period = new PeriodBuilder { Years = 4, Months = 4, Days = 13 }.Build();
LocalDate today = new LocalDate(2013, 2, 8);
LocalDate start = today - period;
Console.WriteLine(start);
Output:
25 September 2008
Note that this doesn't give September 26th, because of the somewhat crazy nature of date/time arithmetic. If you added the period to September 26th you'd get today... but that's not the same thing. Treat this as a warning that you need to be really careful about describing what you want to achieve :)
This second side you can do with the BCL fairly easily though:
DateTime today = new DateTime(2013, 2, 8);
DateTime start = today.PlusYears(-4).PlusMonths(-4).PlusDays(-13);
There's no BCL type to represent that "years, months, days" value though.

Your question is a little confusing. Do you want to subtract one date from another date, or do you want to subtract a period of time from a date.
1. Subtract one date from another date:
DateTime previousDate = new DateTime(1990, 12, 12);
DateTime currentDate = DateTime.UtcNow;
TimeSpan difference = currentDate - previousDate;
You can then use the TimeSpan methods to get the difference in various units of time as you like.
Here's more info on TimeSpan: http://msdn.microsoft.com/en-us/library/system.timespan.aspx
2. Subtract a period of time from a date
DateTime currentDate = DateTime.UtcNow;
TimeSpan periodOfTime = new TimeSpan(12, 12, 0, 0);
DateTime newDate = currentDate - periodOfTime;
However, you'll have to calculate yourself what the length of a month is, if that's what you want.

You can use DateTime.Subtract.
Examples from article:
System.DateTime date1 = new System.DateTime(1996, 6, 3, 22, 15, 0);
System.DateTime date2 = new System.DateTime(1996, 12, 6, 13, 2, 0);
System.DateTime date3 = new System.DateTime(1996, 10, 12, 8, 42, 0);
// diff1 gets 185 days, 14 hours, and 47 minutes.
System.TimeSpan diff1 = date2.Subtract(date1);
// date4 gets 4/9/1996 5:55:00 PM.
System.DateTime date4 = date3.Subtract(diff1);
// diff2 gets 55 days 4 hours and 20 minutes.
System.TimeSpan diff2 = date2 - date3;
// date5 gets 4/9/1996 5:55:00 PM.
System.DateTime date5 = date1 - diff2;

TimeSpan timeSpan = new TimeSpan(2,2,0);
DateTime dateTime = DateTime.Now.Subtract(timeSpan);

When you subtract two date in C# you get a TimeSpan object.
You can acces different properties of it to get the actual days, hours, minutes etc. taht it represents:
DateTime a;
DateTime b;
//assign some values
TimeSpan span = a.Subtract(b);
Console.WriteLine("Days: " + span.Days);

The following should do.
TimeSpan diff = DateTime.Now - previousDate;

How to get a Timespan of 1 year?

I want to get a Timespan structure which represent a year in C#.

The tricky thing is that what a year is, depends on where it starts.
You can do
DateTime now = DateTime.Now;
TimeSpan span = now.AddYears(1) - now;
This would give you the 1 year timespan from the current moment to one year later

The key question here is: which year?
The length of the timespan obviously depends on whether the year you want is a leap year or not and when it starts.
If you want one year starting from today go with #sehe's answer.
If you want the current year go with #Oyvind,
If you want a reasonable approximation you can go with #Nayan, or for a 365.25 approximation use:
TimeSpan oneYearSpan = new TimeSpan(365, 6, 0, 0);

You can't, as a year doesn't have a fixed length (is it 365 or 366 days or about 365.25?). That's also why you can't have a month as TimeSpan (28, 29, 30, 31 days??)

Rough example:
TimeSpan oneYearSpan = new TimeSpan(365, 0, 0, 0);
Will this do?

DateTime intialDate = Date.Now.Date;
TimeSpan yearSpan = intialDate.AddYears(1).Subtract(intialDate)
As other peoplehave mentioned you may want to consider leap years. In that case you can intiate intialDate accordingly.

If you want to be pretty accurate you could use the number of nano seconds in a year.
I think that this moves by 0.5 seconds every century, so should be good for a long while yet!
public TimeSpan MyYear
{
get
{
// Year = 3.1556926 × 10^16 nanoseconds
return new TimeSpan(31556926000000000);
}
}
There are already some good answers on this page, this is just another option.

It depends on which year you want to represent, since not all years are of equal length.
This is the way to find the length of 2010 for example:
var timestamp = new DateTime(2011, 1, 1) - new DateTime(2010, 1, 1);
Change the year in the DateTimes to find the length of the year you want.

Here's how to do this, utilizing the IsLeapYear to determain number of day.
int span = DateTime.IsLeapYear(1996) ? 366: 365;
var year1996 = new TimeSpan(span, 0, 0, 0);

subtract 2 datetime fields to get the days left difference

Would appreciate it if anyone can help me figure out to substract 2 datetime fields to get the days left difference.

This is very easy to do with C#. For comparing DateTimes, we have a class called TimeSpan. The TimeSpan structure, in this case, would be defined as the difference between your two datetimes.
Let's say that your DateTimes are called start and end.
DateTime start = new DateTime(2009, 6, 14);
DateTime end = new DateTime(2009, 12, 14);
We have established our DateTimes to June 14, 2009 and December 14, 2009.
Now, let's find the difference between the two. To do this, we create a TimeSpan:
TimeSpan difference = end - start;
With this TimeSpan object, you can express the difference in times in many different ways. However, you specifically asked for the difference in days, so here's how you can get that:
Console.WriteLine("Difference in days: " + difference.Days);
Thus, the property is called TimeSpan.Days.
Final Code
//establish DateTimes
DateTime start = new DateTime(2009, 6, 14);
DateTime end = new DateTime(2009, 12, 14);
TimeSpan difference = end - start; //create TimeSpan object
Console.WriteLine("Difference in days: " + difference.Days); //Extract days, write to Console.
For more information on using the TimeSpan structure, see this MSDN documentation (especially the C# examples).
Hope I helped!
UPDATE: Some answers have suggested taking doing subtraction in one step, such as with:
int days = (dt2 - dt1).Days;
or
int numDaysDiff = Math.Abs(date2.Subtract(date1).Days);
However, they are the same thing as in my answer, only shortened. This is because the DateTime.Subtract() method and the subtraction operator of DateTimes returns a TimeSpan, from which you can then access the amount of days. I have specifically used the longer approach in my code sample so that you clearly understand what is going on between your DateTime and TimeSpan objects and how it all works. Of course, the other approaches I just mentioned are fine, too.
UPDATE #2:
A very similar question was asked before, and it can be found here. However, the main point of that question was why the code sample (which is essentially equivalent to that of all the answers) sometimes provides an answer which is a day off. I think this is also important to this question.
As the main answer to the other question suggests, you can use this code:
int days = (int)Math.Ceiling(difference.TotalDays);
This code uses Math.Ceiling, which, according to MSDN, is:
Returns the smallest integral value
that is greater than or equal to the
specified double-precision
floating-point number.
How Do You Want to Count the Days?
Thus, we now have an issue with how you want to count the days. Do you want to count part of a day (such as .5 of a day) as:
A full day - this would use Math.Ceiling to round up TimeSpan.TotalDays, so that you're counting started days.
Part of a day - you can just return the TimeSpan.TotalDays (not rounded) as a decimal (in the double datatype)
Nothing - you can ignore that part of a day and just return the TimeSpan.Days.
Here are code samples for the above:
Counting as a full day (using Math.Ceiling() to round up):
//establish DateTimes
DateTime start = new DateTime(2009, 6, 14);
DateTime end = new DateTime(2009, 12, 14);
TimeSpan difference = end - start; //create TimeSpan object
int days = (int)Math.Ceiling(difference.TotalDays); //Extract days, counting parts of a day as a full day (rounding up).
Console.WriteLine("Difference in days: " + days); //Write to Console.
Counting as part of a day (NOT using Math.Ceiling(), instead leaving in decimal form as a part of a day):
//establish DateTimes
DateTime start = new DateTime(2009, 6, 14);
DateTime end = new DateTime(2009, 12, 14);
TimeSpan difference = end - start; //create TimeSpan object
double days = difference.TotalDays; //Extract days, counting parts of a day as a part of a day (leaving in decimal form).
Console.WriteLine("Difference in days: " + days); //Write to Console.
Counting as nothing of a day (rounding down to the number of full days):
//establish DateTimes
DateTime start = new DateTime(2009, 6, 14);
DateTime end = new DateTime(2009, 12, 14);
TimeSpan difference = end - start; //create TimeSpan object
int days = difference.TotalDays; //Extract days, counting parts of a day as nothing (rounding down).
Console.WriteLine("Difference in days: " + days); //Write to Console.

Use
TimeSpan
DateTime departure = new DateTime(2010, 6, 12, 18, 32, 0);
DateTime arrival = new DateTime(2010, 6, 13, 22, 47, 0);
TimeSpan travelTime = arrival - departure;

The easiest way out is, making use of TimeSpan().
This Subtract function will return you the difference between two dates in terms of time span. Now you can fetch fields like days, months etc. To access days you can make use of
Here is the sample code;
VB.Net code;
Dim tsTimeSpan As TimeSpan
Dim ldDate1 as Date
Dim ldDate2 as Date
'Initialize date variables here
tsTimeSpan = ldDate1 .Subtract(ldDate2)
Dim NumberOfDays as integer = tsTimeSpan.days
C#.Net code;
DateTime lDate1;
DateTime lDate2;
TimeSpan tsTimeSpan ;
int NumberOfDays;
//Initialize date variables here
tsTimeSpan = ldDate1 .Subtract(ldDate2);
NumberOfDays = tsTimeSpan.days;

DateTime dt1 = new DateTime(2009,01,01,00,00,00);
DateTime dt2 = new DateTime(2009,12,31,23,59,59);
int days = (dt2 - dt1).Days;

Number of Days Difference
These answers take the number of days as an int from the System.TimeSpan structure that is the result of subtracting two System.DateTime fields...
Quick answer - gets the number of days difference.
int numDaysDiff = date2.Subtract(date1).Days;
Alternate answer - uses Math.Abs to ensure it's not a negative number, just in case the dates might be supplied in either order.
int numDaysDiff = Math.Abs( date2.Subtract(date1).Days );
Some sample data to finish it off using System namespace:
// sample data
DateTime date1 = DateTime.Now;
DateTime date2 = DateTime.Now.AddDays(10);
MSDN References (and more sample code ):
System.TimeSpan structure
System.DateTime structure
System.Math.Abs(..) method

DateTime theDate = DateTime.Today;
int datediff = theDate.Subtract(expiryDate).Negate().Days;
if expiryDate > theDate then you get Negative value: -14
expiryDate is less than theDate then you get positive value: 14
You May obviously want this in a scenario such as
Send a Notification Email 14days before expiry
Send another notification Email 14 days after expiry
You need a difference that could be negative value

You should look at TimeSpan.

To get the exact days ignoring the time section
DateTime d1 = Convert.ToDateTime(DateTime.Now.ToShortDateString());
DateTime d2 = Convert.ToDateTime(DateTime.Now.AddDays(46).ToShortDateString());
var days = Convert.ToInt32(d2.Subtract(d1).TotalDays)