What is the Earliest my Employer can write my sponsor letter?

[Ristoril]

My T2 visa ends September 1st, because that is when my employment switches from Employer A to Employer B.

I can apply for IRL in July, but Employer A can't write the letter since they know I will be "leaving" September 1st (technically im staying in the same place, but the grant for my pay is from Employer B). Is there anyway that Employer B can write the letter before the first of September so I don't have to pay for a full years visa just for 1 day?

If the T2 ends the 1st, can I technically apply for ILR then with a letter written on the first?