Student to Skilled worker ISC exemption time?

[Joraffe23]

Sorry for double posting, i thought maybe i posted in wrong forum last week and i used wrong term in title and couldnt edit.

My Course officially finishes today My BRP end is the end of May

My employer wishes to sponsor me for a skilled worker visa and i want to save them money in hope they will give me 5 years straight away.

They will issue ISC exempt. How long do i have to use the CoS to mean they do not get a letter saying they now need to pay the ISC charge? It has happened to a collegue who waited too long saving funds to apply after being given the CoS. They are still willing to sponsor her but only a year at a time.

Do they need to put the start date of CoS early to stay exempt, or can they put start date in May and still be exempt as long as i apply when they give it on 29/1 ? Does it just depend when i use the CoS to apply?
They have asked helpline who are not helpful

I can apply before the start date can't i?

Thank you