plpgsql & date-time functions

I have a function and I want to update a table's two rows but having problem 
with plpgsql & date-time functions. First field of the table must be now() 
this is ok.. but the second field must be now() + '60 days' if the query 
like this : SELECT INTO to_day now() + interval '60 days' ; it works but if 
I use variable instead of  '60 days' it doesn't work. And I must use 
variable....

Thanks