porting time calcs to PG

I'm trying to port an MS statement that's a bit involved with
timestamps, and I don't see anything in the docs to lead me forward.
It's basically a select statement, looking for records with a timestamp
within a certain range, where that range is calculated with one of the
fields. The WHERE clause that I use in SQL Server is:
getdate() + ((2100 + 5 + (9*Points)) / 86400) >= DueTime

Where the numbers are actually parameters passed in to the function.
Other than changine getdate() to now(), I'm not sure how to change the
+ interval to be effective. All the docs I see use something like
interval '1 hour' - not sure how to put a calculated value in the
quotes. Is this possible?