bash argument case for args in $@

I have a script with a long list of OPTIONAL arguments. some have associated values.

Such as:

.script --first 2012-12-25 --last 2012-12-26 --copy --remove
.script --first 2012-12-25 

Thus the following case statement:

for arg in "$@"
do
    case $arg in
        "--first" )
           START_DATE=$arg;;
        "--last" )
           END_DATE=$arg;;
        "--copy" )
           COPY=true;;
        "--remove" )
           REMOVE=true;;

# ... and so on
   esac
done

My problem:

that needs a increment $arg+1 type statement to get the following arg (in some cases).

How is that possible?

I'm also happy to do a substring such .script --first2012-12-25 --last2012-12-26

and not sure how to proceed there.