Question about extra duty after time is complete?

I have a quick question, I recived extra duty for 7 days an started last Tuesday. I am aware that extra duty starts the night i got read. my question is if i started on tue, that would be concidered day 1 which would make this past monday the 23rd of Aug. my last night. there for completing me extra duty, my PS informed me today that i have to preforum extra duty again tonight that Monday was not my last night. My question is can they extended my extra duty just becuase they feel like it an consider tue to tue seven days even thought it would be 8 days. And could someone point me to a regulation i looked at ar27-10 but could not find the answer