caluculate the rangeof date values and roundoff the output value

need a help on get the roundoff value..
where i am passing parameters is coming like month or day or week or year..
i need to convert those values onto days and caluculate the range then i have to roundoff the values as per the requirement
right now i have inserted the list of days , weeks , months and years in the database table.(but this is not the correct way to do this , for an emergecy i have inserted those values getting the roundoff values..)
created funtion:

CREATE FUNCTION [dbo].[fn_conv_Get_DurationAndUnit]  
(  
 @Duration INT
,@DurationUnit VARCHAR(100)
)  
  
RETURNS VARCHAR(250)  
AS BEGIN  
	DECLARE @ReturnDurationUnit VARCHAR(250)		
	SET @ReturnDurationUnit = (CONVERT(VARCHAR(10),@Duration) + ' ' + LTRIM(RTRIM(@DurationUnit)))	
	RETURN @ReturnDurationUnit
END  

in db insert values like 
input value destination value
1 Weeks	1
10 Days	2
11 Days	2
12 Days	2
13 Days	2
14 Days	2
2 Weeks	2
" " " 
"
15 Days	3
16 Days	3
17 Days	3
18 Days	3
19 Days	3
3 Weeks	3
20 Days	4
1 Month	4
60 Days	12
480 Days	96
96 Weeks	96
24 Months	96

i need this as more dynamic...please let me know if you have any Questions.