They can be stacked ... so you can put on 10 of the 3 day shields and be protected for 30 days ... people do this a lot when they go on vacation.

I have a bank account with me and there is like 48weeks of shield in effect now. Yes shields can be stacked like CIM said. So its normal to have longer shield when you intend to come back in game. One of my friend have some stuff so he is busy with it but he laid 7months of shield just to be back in game anytime he wants and find his army safe. If u want to see how long that shield will be then click on player then press domain guard it will cost you 1 domain guard or 2k gold but will tell you when it gets over.