Your join date is the either the date you were accepted as a member of your first KiwiSaver Scheme, or the date the IRD received your first contribution, whichever is earlier. Your KiwiSaver provider or IRD will be able to confirm your official start date.
If you were over 60 when you joined KiwiSaver, you will be eligible to withdraw from your KiwiSaver Scheme five years after your joining date. You are entitled to member tax credits for that whole five year period.
For example, if you joined in September, your first member tax credit payment will only be a portion of the full $521.43, as the KiwiSaver year runs from 1 July to 30 June. Your final member tax credit payment will also be a partial payment, but it will work out so that you get the full five years’ worth of member tax credits.
More detailed information on how to calculate your member tax credits is available at http://www.gmi.co.nz/pages/kiwisaver/61/Member-Tax-Credits.aspx