Thanks for your help, weird things happen, Natron accepts the expression when typing it in,
but when closing the window a popup appears, saying: invalid expression.
It counts down the time in the “Text Node” text window, but in viewer it stays always on 30:00:01
No error message when entering the expression in text node:
Ubuntu Linux 18.04 LTS, Natron 2.3.15 82693d4