Auto date stamp with drop down list?

I'm producing a new spreadsheet to track issues and thier completion.

I've used the code I have on another spreadsheet, that I got off this site, thanks.

What I would like to change are the following:
1) The status column (F) uses a drop down list, located in cells Z1 to Z5.
When an option is selected, the date updates in column G automatically.
What I would like is for the date to only update in column G if the drop
down list is changed to Completed.

2) If the status is changed to completed by mistake, and then changed back
I would like the date to disappear.

Finally, can I make the date columns jump automatically so the person entering the data goes from column F to column H.

Many thanks,
Gavin.