Unemployment Period between Tier 2 G Visas

[kps16]

Hi, I have been employed in a Tier 2 General visa and Jan 2nd, my employer sent the termination notice due to the revocation of the company's sponsorship license. I have not received the curtailment letter yet. I had been trying for another job and recently cleared one interview and about to submit the visa application. Considering my situation, requested the new employer to process same day visa as I can join the company at any time and they gracefully accepted. They have not given the employment start date. But like to know, when is the earliest date I can start the employment with the new employer if the visa is successful?
1) Visa submission and process date (same day visa service)
2) Brp received date

I understand that CoS start date to be set. When can be the earliest date set?