Arithmetic overflow error converting numeric to data type varchar. while passing large date differance

Arithmetic overflow error converting numeric to data type varchar. while
passing large date differance

my stored procedure:
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 function: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(8), convert(decimal(18,2), @Mns)), 2) + ':' +
right('00' + CONVERT(decimal(10,0), convert(varchar(10), @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: what is wrong with my function