while passing bigger date differnce getting erro in sql

while passing bigger date differnce getting erro in sql

I have stored procedure like this on my DB:
ALTER procedure [dbo].[performance]
(@startdate nvarchar(100), @enddate nvarchar(100)
as begin
declare @date1 nvarchar(100) = convert(varchar, @startdate+'
00:00:00.000', 120)
declare @date2 nvarchar(100) = convert(varchar, @enddate+'
23:59:59.000', 120)
set NOCOUNT on;
select l.LocName,v.Vtype, SUM(DATEDIFF(MI,t.Paydate,t.DelDate)) as
TotalDiff,
[dbo].[testfunction](
CONVERT(decimal(10,1), AVG( CONVERT(NUMERIC(18,2),
DATEDIFF(SS,t.Paydate,t.DelDate) ) ))) as Average
from Transaction_tbl t
left join VType_tbl v
on t.vtid=v.vtid
left join Location_tbl l
on t.Locid=l.Locid
where t.Locid in
(select t1.Locid from Transaction_tbl t1)
and dtime between '' + @date1 +'' and ''+ @date2 +''
and Status =5
group by v.Vtype,l.LocName,l.Locid order by l.Locid
end
my testfunction ike this:
ALTER FUNCTION [dbo].[testfunction] (@dec NUMERIC(18, 2)) RETURNS Varchar(50)
AS
BEGIN
DECLARE
@hour integer,
@Mns integer,
@second decimal(18,3)
DECLARE @Average Varchar(50)
select @hour=CONVERT(int,@dec/60/60)
SELECT @Mns = convert(int, (@dec / 60) - (@hour * 60 ));
select @second=@dec % 60;
SELECT @Average =
convert(varchar(9), convert(int, @hour)) + ':' +
right('00' + convert(varchar(2), convert(int, @Mns)), 2) + ':' +
right('00' + CONVERT(decimal(10,0), convert(varchar(6), @second)), 6)
RETURN @Average
END
if i pass start date:2013-06-01 and end date:2013-08-01 then getting
proper out put if i pass start date:2010-06-01 and end date:2013-08-01
(bigger date difference) then getting error:
Arithmetic overflow error converting numeric to data type varchar.
i know having some problem with my function.but i am not able find out
what is the issue with my function.if any one please help me to find out