That is the earliest date on which you can apply for PR, since holding a residence visa for two years is one of the requirements. Apart from that, you have to fulfil the other criteria. Go to the red button near the bottom of this page
https://www.immigration.govt.nz/new-...-resident-visa, 'Check criteria,' and you will see all of them, including the various possibilities for 'commitment to New Zealand', with the first one being the 184 days in NZ during each of the preceding two 12-month periods, as mentioned by RougeKiwi.
And yes, you can apply in your own right for PR.
https://www.immigration.govt.nz/new-...-resident-visa
Non-principal applicant eligibility for permanent residence
People who were included in someone else's residence application, such as their parent's or partner's, can sometimes be granted a permanent resident visa without that other person becoming a permanent resident first.
For non-principal applicants to be eligible for a permanent resident visa, the principal applicant must hold a permanent resident visa. In some circumstances a non-principal applicant can be granted permanent residence on their own.
If you were the principal applicant’s partner when you were included in their residence application, you may be eligible for a variation of travel conditions or a new resident visa if one the following apply to you:
You and the principal applicant have separated or divorced.
You have a protection order against the principal applicant.
The principal applicant has been convicted of an offence against you or a dependent child.
The principal applicant died.
The principal applicant is a New Zealand citizen now.
Evidence to support eligibility
You’ll need to provide evidence to support your eligibility, which may include:
a certified copy of your divorce decree or dissolution order
evidence of your separation
a certified copy of your protection order
evidence of a conviction for an offence against you or a dependent child
a certified copy of a death certificate.
(Those two old threads quoted don't apply to the same situation as you. They were questions from people who had had partner-sponsored residence, not been a secondary applicant on their partner's application.)