What we do is, for all users in the G/L set up, we give the current month start and end date. End of month we reset for the new month.
Then we have set up for specific users (other users blank) +10 days extra beyond the closing period in the USERS setup. This means that those users can still post beyond the month on a back date for 10 days.
On the 10th we again ensure that the specific users are also not given any prior date posting and period opening then will be on a special request. This helps the control. Specific users are generally finance personnel and 1 or 2 for invoice posting. All others have to post on the current date.
Number series becomes a tad difficult and I would not recommend, as currently the Navision system is not set up to take multiple date numbers. For eg, I have this problem that for the year 2006 I want to take 2006 series and for the year 2007, 2007 series. When I set it up and try to post on the old 2006 date, the system takes the 2007 series.