Ok, thanks for the feedback.
We will improve error management. Generally, tasks has to be defined before beeing used.
In the meantime, you can use:
[Development 0.1] lasts 9 days
[Production 0.1] starts at [Development 0.1]'s end
[Development 0.2] starts at [Development 0.1]'s end and lasts 6 days
[Production 0.2] starts at [Development 0.2]'s end
[Production 0.1] ends at [Production 0.2]'s start