First, when working with dates, store them as dates. Part of the reason that you're having problems is that you aren't storing your dates as dates.The main issue that people run into when trying to solve this problem, is that they try to preserve information that is irrelevant for the problem at hand. Specifically, if you're looking at differences in years, anything more granular than a year is irrelevant.
You also talk about inputs, but then your sample data is a table. I did a self join to get two dates to work with.
/* Step 1: Adjust the whole fiscal year to match the calendar year. DATEADD(MONTH, -3, <datefield>)
Step 2: Find the difference in years between the two adjusted values.
*/
;
WITH Test_Corrected AS
(
SELECT *
FROM #Test
CROSS APPLY ( VALUES( CAST('01 ' + RTRIM(M2) + ' ' + CASE WHEN M2 IN ('Jan', 'Feb', 'Mar') THEN RIGHT(M1, 4) ELSE LEFT(M1, 4) END AS DATE)) ) fy(fy_month_start)
)
SELECT FromDate.fy_month_start AS FromDate, ToDate.fy_month_start AS ToDate, DATEDIFF(YEAR, DATEADD(MONTH, -3, FromDate.fy_month_start), DATEADD(MONTH, -3, ToDate.fy_month_start)), *
FROM Test_Corrected FromDate
INNER JOIN Test_Corrected ToDate
ON FromDate.fy_month_start < ToDate.fy_month_start
ORDER BY FromDate.fy_month_start, ToDate.fy_month_start
Drew