i do not know the annuity start date what do i do?

[leighinmaine1]

the day he retired

[TomK2023]

You can use the first day of the month he received his very first payment.  

The IRS should also accept the first day of the month he retired as a perfectly acceptable estimate as well.

This date is used to determine his age when the payments began. This tells the software how many "expected payments" he has left, which then determines how much of his pension is tax-free (because he made after-tax contributions to the plan years ago).