What NýĠMA says is correct. The time period of the event is based on when convoys can be claimed and started. As long as a convoy mission is active, it can be plundered.

Some players do tend to try and leave their convoys to the very end in order to try and avoid getting plundered, as there are less players actively trying to plunder. However, it does not mean that plundering is not impossible. The way it works is done so the that the event is not abused by trying to circvmvent the plunder system by waiting outside of the period when convoys can be claimed.

There is no problem with this as it is intended. As stated above, convoy missions are what is limited by the event duration, where there is a set time when you cannot take claim anymore missions. Plunder is dependent on available convoy missions. Since convoy missions can still be active outside the time when you can "claim" a mission, this means that players can plunder as long as a convoy mission is active and meets the requirement necessary to be plundered.

This event is intended this way, where the ability to plunder a mission is determined by the active convoy missions so that players do not cir*vent the system by leaving their convoys to the last minute in hopes that plunderers lose the ability to plunder them.