Well, the contract from Yovana's link says effective as of the first day of January 1991..
The real date of the contract is left open, so it is possible it was negotiated and signed any time early or late 1991.
I don't know about contract rules in the music business but maybe it is common to backdate those kind of contracts to the beginning of the year although it was signed later.
Bottom line: We don't know the date of negotiation and signment.