This is working great for me, too!
The only addonI have is the comment above - it computes duration from given seconds…
The script is nearly the same as for the uptime in the systeminfo binding (Community post) (provided also by Robert)- but only nearly:
The systeminfo binding script really calculates system uptime from given minutes (provided by the binding).
It is missing the
// seconds to minutes
i = Math.floor(i / 60)
So you have to use a different script for openhab runtime than for system runtime.
Maybe that helps others not to stumble into the same problem as me…
.