Csharp: Fix typo in LeapYear qhelp

[igfoo]

This PR corrects a typo in the UnsafeYearConstruction.qhelp file where "add/substract" is changed to "add/subtract".

[Copilot]

Pull Request Overview

This PR fixes a spelling error in the C# documentation for the UnsafeYearConstruction query help file.

[Copilot]

The typo has been correctly fixed from 'substract' to 'subtract'.

[github-actions]

QHelp previews:

csharp/ql/src/Likely Bugs/LeapYear/UnsafeYearConstruction.qhelp

Unsafe year argument for 'DateTime' constructor

When creating a System.DateTime object by setting the year, month, and day in the constructor by performing an arithmetic operation on a different DateTime object, there is a risk that the date you are setting is invalid.

On a leap year, such code may throw an ArgumentOutOfRangeException with a message of "Year, Month, and Day parameters describe an unrepresentable DateTime."

Recommendation

Creating a System.DateTime object based on a different System.DateTime object, use the appropriate methods to manipulate the date instead of arithmetic.

Example

In this example, we are incrementing/decrementing the current date by one year when creating a new System.DateTime object. This may work most of the time, but on any given February 29th, the resulting value will be invalid.

using System;
public class UnsafeYearConstructionBad
{
    public UnsafeYearConstructionBad()
    {
        DateTime Start;
        DateTime End;
        var now = DateTime.UtcNow;
        // the base-date +/- n years may not be a valid date.
        Start = new DateTime(now.Year - 1, now.Month, now.Day, 0, 0, 0, DateTimeKind.Utc);
        End = new DateTime(now.Year + 1, now.Month, now.Day, 0, 0, 1, DateTimeKind.Utc);
    }
}

To fix this bug, we add/subtract years to the current date by calling AddYears method on it.

using System;
public class UnsafeYearConstructionGood
{
    public UnsafeYearConstructionGood()
    {
        DateTime Start;
        DateTime End;
        var now = DateTime.UtcNow;
        Start = now.AddYears(-1).Date;
        End = now.AddYears(-1).Date.AddSeconds(1);
    }
}

References

• System.DateTime Struct.