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How to use DateTime to make a countdown calendar in C#

I'm currently learning C# and making some console applications.
I want to create a console app which take two dates as parameters (YEAR/MONTH/DAY).
One for the start, and another one for the end.
I've tried to make a difference between the two of them, but I get the following error message:
"Cannot implicitly convert type 'System.TimeSpan' to 'System.DateTime' [Calendar]"
Here's my code:
static void Main(string[] args)
{
DateTime t = DateTime.Now;
DateTime end = new DateTime(2017, 11, 17);
int result = DateTime.Compare(t, end);
TimeSpan timeSpan = end - t;
DateTime days_left = timeSpan;
Console.WriteLine("Left", timeSpan.TotalDays);
Console.ReadKey();
}
In this version, I enter the end date in the code.
Thanks in advance for your help and for your time,

The following line is the problem:
DateTime days_left = timeSpan;
When you declared timeSpan you gave it the type TimeSpan. On the very next line, you try to assign timeSpan to days_left, which is a variable of type DateTime. This is not possible, as you cannot directly cast one type to the other.
If you think about it, this line doesn't even make sense, as DateTime objects represent a date not a time span. That is what TimeSpan objects are for!
Simply remove that line and your program will compile no problem.
Also, if I may make a suggestion, do not directly subtract DateTimes like you have done here:
var timeSpan = end - t;
Instead use DateTime.Subtract:
var timeSpan = end.Subtract(t);
This is the recommended approach when dealing with the difference between DateTimes, as it offers benefits such as adjusting for different time zones.
Finally, note my usage of the var keyword instead of explicitly declaring the type. This is a common coding convention in C# that I wish I knew as a beginner.
Here is a revised version of your code. Take some tips from it if you want for programs you write in the future:
public static void Main()
{
var currentDate = DateTime.Now; // Descriptive variable names
var endDate = new DateTime(2017, 11, 17);
double remainingDays = endDate.Subtract(currentDate).TotalDays; //TimeSpan stores everything in doubles instead of integers
Console.WriteLine("Days left: {0}", remainingDays); // Use String.Format formatting
Console.ReadLine(); // Use readline so that program only exists when ENTER is pressed
}

Try changing your code to this
DateTime t = DateTime.Now;
DateTime end = new DateTime(2017, 11, 17);
int result = DateTime.Compare(t, end);
TimeSpan timeSpan = end.Subtract(t);

How I can get the difference by time?

i want to calculate a checktime to the time now and get the hours.
I have a string "time" for example...
string t1 = UserParam[12].ToString(); // "9/26"
string t2 = UserParam[13].ToString(); // "14:51\r"
I need them in my Project where I get from the License Server the time from a user and I want to show the difference to now. I want show this in hours.
I want a time how ---> 1 hour(s), 49 minute(s)

I assume t1 is the month and date and t2 is the hour and minute.
You can use DateTime.ParseExact:
string t1 = "9/26";
string t2 = "14:51";
DateTime dt = DateTime.ParseExact(string.Format("{0} {1}", t1, t2), "M/dd HH:mm", CultureInfo.InvariantCulture);
TimeSpan diff = DateTime.Now - dt;
double totalHours = diff.TotalHours; // 0.726172292194444
Edit:
I get a error --> not the right format
Maybe because you have unwanted characters at the end of the string(s) as in "14:51\r". You can use Trim or TrimEnd to remove them. For example:
string t2 = UserParam[13].ToString().TrimEnd(new []{ '\r', '\n' }); // 14:51

You'll want your times in DateTime structures. If you have timestamp information in a string, then you should use DateTime.TryParse to fill a DateTime. But if you have the timestamp in a string it will be important to specify time zone info in the style parameter (DateTimeStyles).
Then you can take make a TimeSpan by subtracting two DateTime stucts and use the TotalHours property
(dt1 - dt2).TotalHours

You can use DateTime.Subtract to calculate the difference between dates and times:
string date = "9/26 14:51";
DateTime dt = DateTime.ParseExact(date, "M/dd HH:mm", CultureInfo.InvariantCulture);
TimeSpan diff = DateTime.Now.Subtract(dt);

DateTime calculation

I have been getting an annoying issues. I have two datetime variables. Date of employment and termination date. I need to get the number of days work. termindation date - date of employment.
how do i go about getting this?
DateTime empDate = int.Parse((employeeEmploy.ElementAt(i).dateofEmpl).GetValueOrDefault().ToString("yyyyMMdd"));
DateTime terminDate = int.Parse((employeeEmploy.ElementAt(i).terminDate ).GetValueOrDefault().ToString("yyyyMMdd"));
int? dWorked = terminDate - empDate;
I tried that but that didnt work

Well, you're trying to deal with DateTime values - so you shouldn't be using int.Parse to start with. Use DateTime.ParseExact. Once you've got two DateTime values, you can use subtraction to get a TimeSpan, and then compute the total days from that:
DateTime employmentDate = ...;
DateTime terminationDate = ...;
TimeSpan employmentDuration = terminationDate - employmentDate;
int days = (int) employmentDuration.TotalDays;
Personally I'd actually use my Noda Time project to do all of this, mind you:
private static LocalDatePattern TextPattern =
LocalDatePattern.CreateWithInvariantInfo("yyyyMMdd");
...
LocalDate employmentDate = TextPattern.Parse(...).Value;
LocalDate terminationDate = TextPattern.Parse(...).Value;
int days = Period.Between(employmentDate, terminationDate, PeriodUnits.Days)
.Days;

Subtracting DateTime objects produces TimeSpan. So, use TimeSpan.TotalDays to get total days count between two dates:
int dWorked = (terminDate - empDate).TotalDays;
UPDATE: For LINQ to Enitites use EntityFunctions.DiffDays method, which calculates days between two nullable dates:
from x in context.Foo
select EntityFunctions.DiffDays(x.FromDate, x.ToDate)

try something along the lines of
var numDays = (terminDate - empDate ).TotalDays;
dworked = (int)Math.Round(numDays, MidpointRounding.AwayFromZero);

You can easily substract the two Datetimes which gets you a TimeSpan!
DateTime employmentDate = new DateTime(2013,03,8);
DateTime terminationDate = new DateTime(2013,03,11);
TimeSpan days = terminationDate - employmentDate;
Console.WriteLine("Days: " + days.TotalDays); //Result: "Days: 3"

How to set current time to a value

I was just wondering if there is a way to get the current time and set it into a value.
If its 12:06 AM.. I want to get that time and set it into currentTime.
Example
float currentTime = 0;
currentTime = 12.06;

As others have mentioned, the DateTime class would be ideal for this, and to work out the difference between 2 date/times:
DateTime end = DateTime.Now;
DateTime start = new DateTime(2011, 12, 5, 12, 6,0);
double hours = (end - start).TotalHours;
The subtraction of DateTime objects results in a TimeSpan object that you can use to see the hours/minutes etc.

try DateTime class
DateTime dt = DateTime.Now;

Is this what you're looking for?
DateTime currentTime;
currentTime = DateTime.Now;

Don't use floats or strings. You can do all kinds of cool things using DateTime.
Here's how you'd get the hours that someone worked:
var clockIn = new DateTime(2011,12,4,9,0,0); // December 4th, 9 AM
var clockOut = new DateTime(2011,12,4,17,0,0); // December 4th, 5 PM
var duration = clockOut - clockIn; // TimeSpan
Console.Write(duration.TotalHours); // 8

A few people have mentioned how, but as a 'better' recommendation you should use
DateTime currentTime = DateTime.UtcNow
Otherwise you have issues when the clocks go back, if your timing code is run on those days. (plus it is far easier to alter the UTC time to local time than it is to convert a '1am' to UTC (as there will be two of them when the clocks go back)

Well if you really what it as a float then try:
var currentDate = DateTime.Now;
float currentTime = float.Parse((currentDate.Hour > 12 ? currentDate.Hour -12 :
currentDate.Hour) + "." + currentDate.Minute);
I wouldn't recommend comparing dates or time with floats. A better options would be to use timespans.

You should be using a Timespan instance for time related values, you can use the flexibility to get the required values like
TimeSpan ts = DateTime.Now.TimeOfDay;
ts.ToString("hh:mm") // this could be what you are looking for
You could then use ts.TotalHours which would give you fractional hours (as a double) else you could construct a string specifically using ts.Hours ..ts.Minutes play around and it could be prove useful.

Try the following:
DateTime StartTime=StartTime value;
DateTime CurrentTime=DateTime.Now;
TimeSpan dt = CurrentTime.Subtract(StartTime);
In dt you will get a working time period.

If you want to have the difference between two times, then do this:
DateTime dateOne = DateTime.Parse(enteredTime);
DateTime dateTwo = DateTime.Now;
TimeSpan difference = dateOne - dateTwo;

Join Date and Time to DateTime in C#

I am retrieving data from an iSeries where there is a separate date and time fields. I want to join them into a DateTime field in my C# project. I don't see a way to add just a time to a DateTime field. How would you suggest accomplishing this?

You can do this quite easily:
DateTime dateOnly;
DateTime timeOnly;
...
DateTime combined = dateOnly.Date.Add(timeOnly.TimeOfDay);
TimeOfDay returns a TimeSpan, which you then add to the date.
Edit (thanks to commenters below) - to be safe, use dateOnly.Date to ensure the date part only.

How are they being stored? Assuming that the date portion is being stored as a DateTime of midnight of the day in question and the time is a TimeSpan, you can just add them.
DateTime date = ...;
TimeSpan time = ...;
DateTime result = date + time;

You could easily construct a TimeSpan from your "time" field.
Once you have that, just do:
TimeSpan time = GetTimeFieldData();
dateField = dateField.Add(time);

Datetime date = new DateTime(Date1.Year, Date1.Month, Date1.Day, Time1.Hour, Time1.Minute, Time1.Second);

You can add a TimeSpan to a DateTime and write something like this.
// inside consuming function
ISeriesObject obj = getMyObject();
DateTime dt = getDate(obj) + getTime(obj);
private DateTime getDate(ISeriesObject obj)
{
//return a DateTime
}
private TimeSpan getTime(ISeriesObject obj)
{
//return a TimeSpan
}

My answer addresses joining two objects of DateOnly and TimeOnly in .NET 6:
DateOnly orderDate = ...
TimeOnly orderTime = ...
DateTime orderDateTime = orderDate.ToDateTime(orderTime);

This should do:
var output = date.Date + time.TimeOfDay;
or
var output = new DateTime(date.Year, date.Month, date.Day,
time.Hour, time.Minute, time.Second);
suppose that both variable date and time are both of Type DateTime

Note that adding the time to the date is not your biggest problem here. As #Reed Copsey mentioned, you just create a DateTime from the date and then .Add the time.
However, you need to make sure that the iSeries date and time (a Unix time most probably) are in the same representation as the .Net representation. Thus, you most probably need to convert it by adding it to a Jan 1, 1970 DateTime as well.

Cant you simply format the date part and time part as separate strings, then join them together? Then you can parse the string back to a DateTime object