Would a preparer be able to fill out 2012 and 2014 forms according to current rules?

I am filling out a tax form (USA) from 2012 and 2014 in order to lock in capital losses from which to offset subsequent capital gains. (I am not asking about whether I should do this…) I am using old brokerage statements to do so and am confused about what dates I should use for date acquired and date disposed when going short stock and options. So I am looking online for a tax preparer. Would they be able to apply the dates by current customs or do I have to find someone with experience back to 2012 who's able to add the dates by those rules? Thank you for your time.