Ok, got it. BPMN2 defines 3 types of timer expressions: timeDuration (trigger after a certain amount of time), timeCycle (trigger repeatedly) and timeDate (trigger on a specific date and time). It seems you're looking for the last one. Unfortunately, only the first two options are currently supported in jBPM5.
I'll look at adding support for this: https://issues.jboss.org/browse/JBPM-3298
As a workaround for now, I think it should be possible to use timeDuration as described earlier, but then add some action script before that that would translate the generated date and time variable back into a duration variable.
Kris