How to automatically update a formula based on current date

I'm trying to create a spreadsheet to keep track of repeat orders. Its set up so that you enter the start date and delivery interval and it calculates the date of next shipping and delivery based on a few variables. This all works fine but I can't figure out how to update the formulas once the date of next delivery has passed. Ideally I'd like it to work the next shipping and delivery date (which it does) but then when that shipping date has passed it automatically then calculates the next date. I've attached a copy of the spreadsheet and formulas I'm using but can't figure out to make this automatic update. Obviously If I manually change the start date every time the shipping date passes it would work out the next date but is there a way to do it automatically. Thanks in advance.

Paul

Ok so lets say for example that the start date is 01/04/14 at 7 day interval it would show a next delivery date of 08/04/14. Once the 08/04/14 has passed the start date would update to 08/04/14 so that it then shows the next delivery date of 15/04/14 etc.

Thanks

OK thanks. I've no experience of programming macros:-( would anyone be able to help with some code please?