In Civilization War there are several items that can add an extra buff effect to certain game parts. 

Their effect duration has a time limit, which is stated in both the name of the item and its icon.

Some examples of such items are: War Protection Shield, Activate VIP, Decrease Food Consumption, Block Scouting, etc.

It is important to remember that if you use the same time-limited item before the effect duration of the previous one is over, their effects will not be stacked.

Instead, the new effect duration will be applied, while the previous item's remaining time will be reset.

Ex.) You had a War Shield applied with 2 hours remaining →  used a new 24-hour War Shield item → War Shield duration is now 24 hours (not 26 hours)

Like with all other buff items, their effects are only applied to the actions initiated after item usage (when applicable).