Derivation
variable: stillmardate (table[s]: all_spouse_pairs_full [derived table])
<after all the summary dates are finalised the earliest date either of the partners reported being married to the other partner that is on or after mardate or fstknwmardate is taken:>
egen start=rowmin(mardate fstknwmardate)
bys wife husband: egen temp1=min(stillmardateO) if stillmardateO~=. & stillmardateO>=start
bys wife husband: egen fstmarrepdate=max(temp1)