accounting period lock to avoid wrong periods

although (or because) I have opened the date field for free data entry and removed the automatic setting (today) it's still a continous source of hazzle that people enter dates of wrong periods which can't be corrected any more

So I propose to add a field "current period" to user preferences and check date entries against this.
if the user has entered a date not in the current period the following choices should be available:
* go back and enter another (correct) date (default)
* confirm the exception - (I know this is not a hard check - but that's reality)
* confirm and set the current period according to the new date.