Make tasks Duration automatically change with date changes

[Jedra]

So I'm having a problem and I'm sure it's becuase



a) I lack understanding and
b) I'm using project in a slightly weird way

I have a plan that I'm using to track delivery dates for different products from projects across my programme. I don't know (or care about) the days effort that a specific product will take. The only information I need/want is:

1) When is it scheduled to start
2) When is is scheduled to finish
3) What other products need to finish before it can start

This will allow me to see if delays in delivering a product will delay a programme-level product or objective.

The problem I'm having is that reports come to me in the form of "Product Y will be finished on June 30th" (when it was previously a different date). When I then change that date in project for a product that is not the first task in a chain, it keeps the duration of the task fixed but moves the start date.

For example (forgive the ASCII art):

¦First Product 20 days ¦-\/
¦2nd product 20 days ¦


Then if I change the DATE the 2nd product will be completed it changes to:

¦First Product 20 days ¦-----------\/
¦2nd product 20 days ¦

I want it to change to:

¦First Product 20 days ¦-\/
¦2nd product 30 days ¦

I've tried the different task types and it seems to do it on pretty much any of them.

Any idea how I can resolve this?

(I know I could just change the days but this is a big plan with a lot of updates and potential updaters and I need to make the update process as quick, easy and intuitive as possible).