Calculate two items in new item

I have set two Number items

Number Durration "[%s]" {channel="....."}
Number CurrentTime "[%s]" {channel="....."}

Now i want have a Number Total like this:

Number Total [Durration-CurrentTime]

Givs a change to make this?

I found a solution with rules: Simple + calculation in items (?)
is this the only way?

Create a rule for that. It should be triggered on changes of the two items and calculate whatever you want to postUpdate the third item.
Yes, a rule is the way to go!