Tier 4 Working Hours - What Counts As A Week?

So on my visa, I can work for 20 hours a week during term time. I got hired at a bookshop for part-time work (which is great, I spent 10 years as a bookseller in the US, so it's like riding a bike)...BUT...the company's work week is Friday-Thursday (inclusive).

Does the government recognize a week as 7 consecutive days or as a full calendar week? I've spent a lot of time on hold with Immigration (and paying for that time on hold) to never actually speak with anyone who can answer my question.

Does anyone know the answer to this?

To me, a week is 7 consecutive days.  Regardless of the way your workweek is organised (Fri-Thurs vs Mon-Sun or Sat-Fri), you should only be working 20 hours in whatever period it is -- I really don't think it matters that the company's work week is Fri-Thurs. Just ensure that it's </= 20 hours during each scheduled period. 

I understand your rationale though, and can appreciate the desire to conform to all rules.  But personally, I would try not to overthink this one. 

I know, it seems like a really insignificant thing to be worried about, but you know how governments work...a really tiny thing and then BAM! you're on the deportation plane home.

Has anyone on this forum got any experience in this? I'm really worried my boss will be angry at me if I don't find and answer soon. :/