How to discover financial Year based on current datetime?

A couple of Extension methods

public static class DateTimeExtensions
{
    public static string ToFinancialYear(this DateTime dateTime)
    {
        return "Financial Year " + (dateTime.Month >= 4 ? dateTime.Year + 1 : dateTime.Year);
    }

    public static string ToFinancialYearShort(this DateTime dateTime)
    {
        return "FY" + (dateTime.Month >= 4 ? dateTime.AddYears(1).ToString("yy") : dateTime.ToString("yy"));
    }
}

I've done the before by creating a FinancialYear class:

public class FinancialYear
{
    int yearNumber;
    private static readonly int firstMonthInYear = 4;

    public static FinancialYear Current
    {
        get { return new FinancialYear(DateTime.Today); }
    }

    public FinancialYear(DateTime forDate) 
    {
         if (forDate.Month < firstMonthInYear) {
             yearNumber = forDate.Year + 1;
         }
         else {
             yearNumber = forDate.Year;
         }
    }

    public override string ToString() {
        return yearNumber.ToString();
    }
}

Other points:

• Have a look at IFormatProvider to see how you can customize formatting (you could provide an overload of ToString that takes a format argument like DateTime does.
• override Equals, and implement IEquitable to provide equality.
• Implement IComparable to give comarisons.
• You could also implement your own == < > >= and <= operators for the class.