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Converting short? to TimeSpan

I'm trying to subtract short? Days from DateTime? InitialDate to get DateTime FinalDate.
I'm not sure if, because they are nullable objects, this is causing problems with the comparison. I've tried casting days to a TimeSpan without success.
short? Days = 7;
DateTime? InitialDate = new DateTime(2012, 10, 5);
DateTime FinalDate = InitialDate - Days ;
//FinalDate should be 5/3/2012 12:00 AM
FinalDate should be 5/3/2012 12:00 AM.

Since both the variables you are dealing with are nullable types, you have to make use of the .Value property of them to access its value. You can also make use of the .HasValue property to check whether it is having any value.
Now comes the subtraction part, I'm not sure what is the role of TimeSpan that you mentioned in the question. But from the output specified and the name of the variable, I assumed that its the number of days. If my understandings are correct, then you can try the following code to subtract the Days from InitialDate to get the FinalDate
For a safe side, you can check whether the InitialDate has a value or not before accessing them.
DateTime? FinalDate = InitialDate.HasValue ? InitialDate.Value.AddDays(-(Days.HasValue? Days.Value : 0)) : null;
This Example may help you to understand things more clear.

You can just subtract the Days value from the InitialDate using the AddDays method, passing in the negative value of Days. Since you don't show what you want to happen it either value is null I've made FinalDate nullable:
short? Days = 7;
DateTime? InitialDate = 5/10/2012 12:00 AM;
DateTime FinalDate? = null;
if(Days.HasValue && InitialDate.HasValue)
{
FinalDate = InitialDate.Value.AddDays(-Days.Value)
}

Find TotalDays between two Datetime? variables

DateTime? arrival = (DateTime?)(t.ArrivalDate.Value);
DateTime? departure = (DateTime?)(t.DepartureDate);
Okay i know both of them are nullable and .TotalDays does not work on nullable object. So kindly tell me how am i supposed to find days difference between these two objects.
Note:
Both objects contains Date(s) i.e. are not null

Since there's no meaningful value to their difference if any of them is null, you only need to concern yourself with the case where they're not:
DateTime? arrival = (DateTime?)(t.ArrivalDate.Value);
DateTime? departure = (DateTime?)(t.DepartureDate);
double? totalDays = arrival.HasValue && departure.HasValue
? (double?)(departure - arrival).GetValueOrDefault().TotalDays
: null;
The subtraction should work because of implicit casting to DateTime.

Note: Both objects contains Date(s) i.e. are not null
If you are sure that dates never have null then you can use .Value for nullable DateTime objects. You will get exception when any of them is null.
double days = departure.Value.Subtract(arrival.Value).TotalDays;

//Set dates
DateTime? beginDate = DateTime.Now;
DateTime? endDate = DateTime.Now.AddDays(10);
//Check both values have a value (they will based on above)
//If they do get the ticks between them
long diff = 0;
if (beginDate.HasValue && endDate.HasValue)
diff = endDate.Value.Ticks - beginDate.Value.Ticks;
//Get difference in ticks as a time span to get days between.
int daysDifference = new TimeSpan(diff).Days;

Here i give you tested code please have a look :
DateTime? startDate = DateTime.Now;
DateTime? endDate = DateTime.Now.AddDays(5);
long differenceOfDays = 0;
if (startDate.HasValue && endDate.HasValue)
differenceOfDays = endDate.Value.Ticks - startDate.Value.Ticks;
int daysDifference = new TimeSpan(differenceOfDays).Days;

Set an empty DateTime variable

I would declare an empty String variable like this:
string myString = string.Empty;
Is there an equivalent for a 'DateTime' variable ?
Update :
The problem is I use this 'DateTime' as a parameter for a 'StoredProcedure' in SQL.
E.g:
DateTime? someDate = null;
myCommand.Parameters.AddWithValue("#SurgeryDate", someDate);
When I run this code an exception is catched telling me the 'StoredProcedure' expected a '#SurgeryDate' parameter.
But i provided it.
Any idea why?

Since DateTime is a value type you cannot assign null to it, but exactly for these cases (absence of a value) Nullable<T> was introduced - use a nullable DateTime instead:
DateTime? myTime = null;

No. You have 2 options:
DateTime date = DateTime.MinValue;
This works when you need to do something every X amount of time (since you will always be over MinValue) but can actually cause subtle errors (such as using some operators w/o first checking if you are not MinValue) if you are not careful.
And you can use Nullable:
DateTime? date = null;
Which is nice and avoids most issues while introducing only 1 or 2.
It really depends on what you are trying to achieve.

You can set a DateTime variable to be '1/1/0001 00:00:00' but the variable itself cannot be null. To get this MinTime use:
DateTime variableName = DateTime.MinValue;

You may want to use a nullable datetime. Datetime? someDate = null;
You may find instances of people using DateTime.Max or DateTime.Min in such instances, but I highly doubt you want to do that. It leads to bugs with edge cases, code that's harder to read, etc.

The method you used (AddWithValue) doesn't convert null values to database nulls. You should use DBNull.Value instead:
myCommand.Parameters.AddWithValue(
"#SurgeryDate",
someDate == null ? DBNull.Value : (object)someDate
);
This will pass the someDate value if it is not null, or DBNull.Value otherwise. In this case correct value will be passed to the database.

Either:
DateTime dt = new DateTime();
or
DateTime dt = default(DateTime);

If you set the date to
DateTime dNewDate = new DateTime();
The value is set to {1/1/0001 12:00:00 AM}

Option 1: Use a nullable DateTime?
Option 2: Use DateTime.MinValue
Personally, I'd prefer option 1.

A string is a sequence of characters. So it makes sense to have an empty string, which is just an empty sequence of characters.
But DateTime is just a single value, so it's doesn't make sense to talk about an “empty” DateTime.
If you want to represent the concept of “no value”, that's represented as null in .Net. And if you want to use that with value types, you need to explicitly make them nullable. That means either using Nullable<DateTime>, or the equivalent DateTime?.
DateTime (just like all value types) also has a default value, that's assigned to uninitialized fields and you can also get it by new DateTime() or default(DateTime). But you probably don't want to use it, since it represents valid date: 1.1.0001 0:00:00.

There's no such thing as an empty date per se, do you mean something like:
DateTime? myDateTime = null;

The .addwithvalue needs dbnull.
You could do something like this:
DateTime? someDate = null;
//...
if (someDate == null)
myCommand.Parameters.AddWithValue("#SurgeryDate", DBnull.value);
or use a method extension...
public static class Extensions
{
public static SqlParameter AddWithNullValue(this SqlParameterCollection collection, string parameterName, object value)
{
if (value == null)
return collection.AddWithValue(parameterName, DBNull.Value);
else
return collection.AddWithValue(parameterName, value);
}
}

This will work for null able dateTime parameter
. .
SearchUsingDate(DateTime? StartDate, DateTime? EndDate){
DateTime LastDate;
if (EndDate != null)
{
LastDate = (DateTime)EndDate;
LastDate = LastDate.AddDays(1);
EndDate = LastDate;
}
}

How to cast to System.Timespan?

can someone tell me how to cast System.Timespan? to System.Timespan
I keep getting this error when tryin to get the date difference between the current date and a date from a linq query (see belwo)
System.TimeSpan ts = i.joinDt - DateTime.Now.Date;

System.TimeSpan ts = (i.joinDt - DateTime.Now.Date).Value;

To get a TimeSpan from a TimeSpan? you need to access the Value property of the nullable - no need to cast.
TimeSpan? tsn = i.joinDt - DateTime.Now.Date;
TimeSpan ts;
if(tsn.HasValue)
{
ts = tsn.Value;
}
Or:
if(i.joinDt.HasValue)
{
TimeSpan ts = i.joinDt.Value - DateTime.Now.Date;
}

can someone tell me how to cast System.Timespan? to System.Timespan
You need to specify a default value for the case where the TimeSpan? is null:
TimeSpan? nullableTs = ...
TimeSpan ts = nullableTs ?? TimeSpan.Zero;

Convert the TimeSpan datatype to DateTime?

I'm converting a small MSAccess application to a web-based ASP.NET app, using C# 3.5. I was wondering what's the best way to work with dates in C#, when converting some of this VBA code over to C#.
Here is an example of the VBA Code:
Coverage1=IIf(IsNull([EffDate1]),0,IIf([CurrDate]<=[EndDate1],[CurrDate]-[EffDate1],[EndDate1]-[EffDate1]+1))
Here is what my current C# code looks like with the errors denoted in the commented code:
public DateTime CalculateCoverageOne(DateTime dateEffDateOne, DateTime dateCurrentDate, DateTime dateEndDateOne)
{
if (dateCurrentDate.Date <= dateEndDateOne.Date)
{
return null; //Get "cannot convert null to System.DateTime because it is a non-nullable value type" error
}
else
{
if (dateCurrentDate.Date <= dateEndDateOne)
{
return dateCurrentDate.Subtract(dateEffDateOne); //Gets error "cannot implicitly convert system.timepsan to system.datetime
}
else
{
return dateEndDateOne.Subtract(dateEffDateOne.AddDays(1)); //Gets error "cannot implicitly convert system.timepsan to system.datetime
}
}
}

cannot convert null to System.DateTime because it is a non-nullable value type" error
The DateTime type is a value type, which means that it cannot hold a null value. To get around this you can do one of two things; either return DateTime.MinValue, and test for that when you want to use the value, or change the function to return DateTime? (note the question mark), which is a nullable DateTime. The nullable date can be used like this:
DateTime? nullable = DateTime.Now;
if (nullable.HasValue)
{
// do something with nullable.Value
}
cannot implicitly convert system.timepsan to system.datetime
When you subtract a DateTime from another DateTime, the result is a TimeSpan, representing the amount of time between them. The TimeSpan does not represent a specific point in time, but the span itself. In order to get the date, you can use the Add method or the Subtract method overload of a DateTime object that accepts a TimeSpan. Exactly how that should look I can't say, since I don't know what the different dates in your code represent.
In the last case, you can simply use the return value from the AddDays method, but with a negative value (in order to subtract one day, instead of adding one):
return dateEffDateOne.AddDays(-1);

It looks like your VB is actually returning a time span, presumably in days. Here's the closest direct translation:
public TimeSpan CalculateCoverageOne(DateTime EffDate1, DateTime CurrDate, DateTime? EndDate1)
{
return (EndDate1 == null) ? TimeSpan.Zero :
(CurrDate < EndDate1) ? (CurrDate - EffDate1) :
(EndDate1.AddDays(1) - EffDate1);
}
If instead you just wanted a count of days, just return the TimeSpan's Days property:
public int CalculateCoverageOne(DateTime EffDate1, DateTime CurrDate, DateTime? EndDate1)
{
return ((EndDate1 == null) ? TimeSpan.Zero :
(CurrDate < EndDate1) ? (CurrDate - EffDate1) :
(EndDate1.AddDays(1) - EffDate1)).Days;
}
And for good measure, this is how I would clean up your final version:
public int CalculateCoverageOne(DateTime dateCurrentDate, DateTime dateEffectiveDate, DateTime dateEffDateOne, DateTime dateEndDateOne)
{
TimeSpan ts;
if (dateEffDateOne == DateTime.MinValue)
{
ts = TimeSpan.Zero;
}
else if (dateEffectiveDate <= dateEndDateOne)
{
ts = dateCurrentDate - dateEffDateOne;
}
else
{
ts = (dateEndDateOne - dateEffDateOne) + new TimeSpan(1, 0, 0, 0);
}
return ts.Days;
}

Get the TimeSpan, then subtract that from the DateTime to get the date you want. For your inner IF statement, it would look like this:
TimeSpan estSpan = dateCurrentDate.Subtract(dateEffDateOne);
return dateCurrentDate.Subtract(estSpan);
EDIT: You may also want to return DateTime.MaxValue and have the calling function check for the max value, instead of returning null.

DateTime is a value type. So, you cannot assign null to DateTime.
But you can use a special value like DateTime.MinValue to indicate whatever you were trying to indicate by null.
DateTime represents a date (and time), like "July 22, 2009". This means, you shouldn't use this type to represent time interval, like, "9 days". TimeSpan is the type intended for this.
dateCurrentDate.Subtract(dateEffDateOne) (or, equivalently, dateCurrentDate-dateEffDateOne) is a difference between two dates, that is, time interval. So, I suggest you to change return type of your function to TimeSpan.
TimeSpan is also a value type, so you could use, for instance, TimeSpan.Zero instead of null.

After some excellent answers (I've up-voted you guys), I've finally hammered out what I think is my answer. Turns out that returning an int, as the number of days, is what worked for me in this situation.
Thanks everyone, for providing your awesome answers. It helped me get on the right track.
public int CalculateCoverageOne(DateTime dateCurrentDate, DateTime dateEffectiveDate, DateTime dateEffDateOne, DateTime dateEndDateOne)
{
//Coverage1=
//IIf(IsNull([EffDate1]),0,
//IIf([CurrDate]<=[EndDate1],
//[CurrDate]-[EffDate1],
//[EndDate1]-[EffDate1]+1))
if (dateEffDateOne.Equals(TimeSpan.Zero))
{
return (TimeSpan.Zero).Days;
}
else
{
if (dateEffectiveDate <= dateEndDateOne)
{
return (dateCurrentDate - dateEffDateOne).Days;
}
else
{
return (dateEndDateOne - dateEffDateOne).Add(new TimeSpan(1, 0, 0, 0)).Days;
}
}
}