Health charge for dependent of British Citizen

[siavashes]

Dear All

hi,

I would be grateful if you help me on below matter.

I used to be T1 ent from 2012-2015 when I got my ILR through T1ent and became British on 2016.

1) my wife came from back home on May 2014 as a dependent of T1ent. and her visa due to expire on May 2018.
2) on May 2019 she would be eligible to apply for ILR as she completing 5 years rule.
3) I have an issue with her current IHS application as there is a question there, asking You're joining or remaining with this person in the UK? their Visa expiry date ?????
obviously Im not on visa anymore and became British on 2016. so what date should I put for above question? the purpose of the question is to calculate the IHS charge, and my wife only needs 1-year visa before she would eligible for ILR.
so
should I answer with: eg.
1) 15/05/2019 and pay £100
2)15/05/2021 and pay £600

I would be grateful if you justify your answer with the relevant rule.

[siavashes]

any thought

[CR001]

A Tier 1 dependent visa will be granted for 3 years so you have to pay £600 IHS.