Convert the TimeSpan datatype to 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);

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.

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;
}