Invoice Queue Sequence

I work in an Accounts Payables department. I get a daily report where certain invoices appear due to some type of issue. I then have to combine these reports so I can determine for how long (how many days) they appeared in this queue.

In column E, I have inserted the invoice number and what I'll call sequence they were in the report. Meaning, if an invoice was on the report from 1/5/17 thru 1/8/17, they would be in sequence A or 1. (I'll be adding in another formula for the min and max date for the sequence, so I don't need help with that part). I'm trying to add in a formula so this won't be a manual process. The formula will be in column C, which I don't know how to make it work and so I input the intended output in Column E. I only added up to three sequences of events for one of the invoice, but it could actually have more, so the -A could actually be -1 and then -2 and so on. I kind of color coded this to maybe make more sense of it

Does any of this make any sense where anyone could offer any assistant? If not, (which it probably doesn't) I'll do my best to try to answer any of your questions.