If conditional approval happens after original expiry date, then new expiry date is 5-6 years from date of interview approval to birthday
If conditional approval happens before original expiry date, then new expiry date is 5 years from original expiry date
Not saying above is the rule, but looks like is the case based on reading thru the threads.
Based on my experience, it's the final approval date (generally the same as the interview date, or the next business day), not the date of conditional approval, that determines the membership expiration date for new applicants, which is five birthdays from the interview date. If you interview on your birthday, I believe it's five (not six) years from that day.
For renewals, the new expiration date is five years from the previous one -- it doesn't depend on when the interview happens. Unless there is a mistake or inconsistency, which can certainly happen. For this reason, some NEXUS members let their membership expire and then re-apply as a new member to get an additional year of membership for the $50 they pay, but then your membership lapses until your new application is approved and you are interviewed, which for most of us is not worth it.
For adults renewing five years from their last interview with no major changes to their application information, the interview is often waived. That is, application status goes from "Pending Review" to "Approved" rather than to "Conditionally Approved", and you receive your new card in the mail shortly thereafter. For children and for adults who didn't interview for their last renewal (or whose information has changed), an interview is typically required. They like to take a new photo and look at your new passport.