Kent,
You must make sure that your variable actually exists in your PSM document. So if you put @stopdate@ in your rtf, the variable must exist in the PSM document already.
The best way is to create a persistent variable. In your Variables window, right-click and choose "Add Variable". Give it the name stopdate. Then, you can right-click on it and say "Edit Expression", and enter whatever expression that you need. The variable is then created, without it being placed in any text boxes. Next time you load up your rtf, it should put the value of the variable where @stopdate@ is.
Hope that helps!
Regards,
Raphaël Lalonde Lefebvre