Update A Computed Field

dear sirs,
this is my first contribution at you reputable forum ..

I have a serious problem which is .. I want to create a trigger which runs at the begining of each month .. so I created a table which has a field --> formula --> month(getdate()) i tested it by changing the month in the windows calender and each time i change the month it is updated in the table when I crated a trigger i checked if update(sal_month)
the trigger is as follows
========================================
create trigger tr_update_loan
on salary_month
for update
as
if update(sal_month)
begin
update hr_emp_loan
set payed = payed + one_loan , rest = rest - one_loan
end
=========================================
it returned an error "Column 'sal_month' cannot be used in an IF UPDATE clause because it is a computed column."
I tried to not to use if update() but when I change the date in the calender it is not updated in the other table

can any one help my in this because I'm doing my graduation project