Valid date for ILR application submission

Hi there, I've got a question on dates for my ILR.

So my eligibility for ILR is coming up next year on a spouse visa on the 5 year route. I initally came out here on a T5 youth mobility visa (aussie) and started living with my partner immediately so we qualified for a spouse visa and I moved onto that directly after my T5 (2 years living together)
.
I received my approval letter for the spousal visa on the 18th of March 2017 and have been living continuously on this visa since this date, successfully recieving my renewal after the initial 2.5 years.
My current BRP expires on the 22nd of May 2022.

Can I apply for my ILR 28 days before the 18th of March 2022 (the 5 years from my initial spouse visa approval letter) or must I wait till 28 days before the 22nd of May 2022 (my BRP expiry date)?

Thanks so much, it's very appreciated!
Meg

What was the start initial date on your first spouse visa brp card?

So after digging through old pics for this it looks like the date of issue was:
16-03-17

So sounds like I can apply 28 days before this date?

Your visa expiry date is irrelevant. It is 28 days from completing your 60 months which will be in Feb 2022