Do not confuse the published operational hours of an aerodrome with the validity periods of the associated ATZ. They are not the same. An ATZ is to be considered in effect whether military or civil and whether the aerodrome is operating within its published hours or it is not. You will not find any legislation or rules defining this.
There are many things established in the good old days requiring only common sense (sometimes called custom and practice). We have a new breed about us that appear to require Sir to make defined rules to abide by. There are no laws/rules that prevent an aerodrome operator, again whether military or civil, from operating in a limited way outside its published hours. If you cannot assure yourself beyond doubt that there is no activity taking place within an ATZ then stay out and avoid.