I have used three formulas to get the FY period.

{@CurrentMonthStart}
//1st of Current Month
Date(Year(CurrentDate),Month(CurrentDate),1)

{@FY Start}
if month({@CurrentMonthStart}) in 1 to 3 then
    Date(Year({@CurrentMonthStart})-1,4,1)
else
    Date(Year({@CurrentMonthStart}), 4, 1)

{@FY End}
if month({@CurrentMonthStart}) in 1 to 3 then
    Date(Year({@CurrentMonthStart}),4,1)-1
else
    Date(Year({@CurrentMonthStart})+1,4,1)-1

If you run the report as of today (Jan 2014), it will return the FY period from April 1st 2013 to March 31st 2014.