DateDiff: How soon is now() ?

For a grad school project we need to build a system that bills people by the amount of time they rent a car. Thus, I needed to use the DateDiff function of VB and SQL in Access. I looked it up using Access help and got to the right MSDN page:

Syntax

DateDiff(interval, date1, date2[, firstdayofweek[, firstweekofyear]])

They tell you that interval is a String and link to what a string is. The part they don't say explicitly is that you need to enclose the interval value in quotation marks. "Of course," you say, "It's a string." Is it too much to ask for an example showing it in use with the quotation marks? e.g.

In Microsoft Access:
WHERE (((Rentals.CarOut)=True)) AND ((DateDiff("h",Rentals.DropOffDate,Now())>3));
In Microsoft SQL: (Transact-SQL for SQL 2000/2005)
WHERE (((Rentals.CarOut)=True)) AND ((DateDiff("h",Rentals.DropOffDate,GETDATE())>3));

Where h is the code for hours.

I just wanted to write this down someplace before I forget it. And why must Access and T-SQL be different? Now() doesn't work in T-SQL and GETDATE() doesn't work in Access.