You're on the right track. Do what Mark said. Here's a bit more explanation of why.
roDateTime is a component. When you create an instance of a component, e.g. dt = CreateObject("roDateTime"), you end up with an object, so dt would be an object instance of the roDateTime component. Components implement interfaces. An interface provides functionality to a component through the use of methods. If you look at the roDateTime component documentation, you'll see that it implements the ifDateTime interface, which is where you'd look to find out about the functionality implemented by the roDateTime component. The interface documentation lists the methods it supports, for example, AsSeconds().
The important thing about using interface methods is that they have to be called on an
object, by specifying the object's identifier followed by a period, the interface name, and a set of parentheses containing parameters, if any.
So to get the number of seconds from the Unix epoch for an roDateTime, you'd do something like this:
dt = CreateObject("roDateTime")
numberOfSeconds = dt.AsSeconds()
To find the difference in seconds between two roDateTime objects, you could do this:
differenceSeconds = dt2.AsSeconds() - dt1.AsSeconds()