Try this

DECLARE @FrmDate date = '31dec2009'

,@EndDate date = '01jan2010'

SELECT

CASE WHEN DATEpart(MM,@FrmDate)=DATEpart(MM,@EndDate)

THEN CASE WHEN DATEpart(DD,@FrmDate)>DATEpart(DD,@EndDate)

THEN datediff(MM,@FrmDate,@EndDate)/12 - 1

ELSE datediff(MM,@FrmDate,@EndDate)/12

END

ELSE datediff(MM,@FrmDate,@EndDate)/12

END