In Company Maintenance on the Preferences tab, you can select the "Use Document Date Warning" option, then  define the number of prior periods and future periods allowed outside the current period, or keep both options at 0 to only allow dates to be entered that are within the current period. 

This new feature works great (as a warning, not hard block)... but doesn't control posting dates (which are only limited by not being able to post to a closed period).

Although, there is a nice warning now if you really typo the year...