You would either have to look back in your records or contact the plan administrator.
Please see tis answer from ToddL
The "plan cost at annuity start date" refers to your total after-tax contributions in the plan, if applicable.
If you did not make any after-tax contributions into these accounts, enter "0" (zero) in the plan cost entry.
If you did make after-tax contributions and have a 1099-R, it may be listed in box 9b. If the information is not listed there, you must find it in your records or contact your annuity administrator. It will be beneficial to find this information, as it may reduce your taxable income.
I withdrew money from my Annuity account and Turbo Tax wants the start date of the Annuity. Is that the date I rolled the money over to the current Annuity holder from another Annuity holder, or do I have to keep going back. I think current Annuity holder is third one that has held the money.
If you are receiving annual payments from the annuity, the annuity start date is the date you received your first payment. If you do not have that date in your records, you may have to contact the annuity holder that was applicable at the time of that first payment.
It is not related to when you rolled the money to a new annuity holder.
The annuity that I withdrew funds from was moved to current holder from another one. I moved the funds to that (2nd) annuity holder from another (3rd) annuity holder. I moved funds to that (3rd) annuity holder from funds I received when I left a company were they were in a 401K (I think) and transferred to an annuity so I wouldn't have to pay taxes on the funds. All this occurred over a period of 25 years or so. So, how do you determine the start date?
However, using the start date of the last annuity holder in TurboTax seems to be acceptable. TurboTax uses the full amount of the withdrawal as a taxable amount. I believe that should be correct.