Syntax for defining an attribute expression subtracting time

We are defining GlDate attribute of a service mapping with an expression, it needs to be 6 hours before the current date/time.

IF we set the expresion as: now()

it works, but we dont know the syntax to reduce 6 hours from now().

We tried:

now()-6 - but it subtracts 6 days

now()-.025 - but it shows an error (see image below)

SYSDATE - INTERVAL '6' HOUR - this shows like 46+ errors