Sql Datetime in where clause?

I am using a time field in my where clause that is returning the proper results..except for April and May below is the query?

declare @sdate date
declare @edate date

set  @sdate  = dateadd(YY,-1,GETDATE())
set  @edate  = GETDATE()

select @sdate, @edate

select datepart(m,recorddate),SUM(qty_invoiced)
from   inv_item
where item = 'f7000356'
and    tax_date between @sdate and @edate
group by  datepart(m,recorddate)



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When i run the below the results are correct if i change the month to  05 again the results are correct?

 


select datepart(m,tax_date),SUM(qty_invoiced)
from   inv_item
where item = 'f7000356'
and    tax_date       between '04/01/2012' and '04/30/2012'
group by  datepart(m,tax_date)



Please help.