I believe, you want difference in year, month and date between two days,
The query will go like
select (datepart(yyyy,@firstDate) - datepart(yyyy,@secondDate)) as Diffyear
select (datepart(mm,@firstDate) - datepart(mm,@secondDate)) as DiffMonth
select (datepart(dd,@firstDate) - datepart(dd,@secondDate)) as DiffDays
If you want better user experience for the user, you may want to check whether first date is greater than second date before above query, if second date is greater than first date, you should re-arrange the query to avoid showing -1 year or -2 month or -15 days kind of display.
Hope that helps