Period is part of Era. Period is related to Century. 
you are likely to find Number in/at/on Store. you are likely to find Stuff in/at/on Store. Stuff is related to Past. Past is the opposite of Actual. 
Era <--PartOf-- Period <--RelatedTo--> Century 
Number --AtLocation--> Store <--AtLocation-- Stuff <--RelatedTo--> Past <--Antonym--> Actual 