The agreement also stated a period of transition for the Baltic countries to last until December 31, 1999. This was a much shorter period than that which was assigned to the Central European countries (their transition periods were declared to be 10 years). Estonia declined the transition period, Latvia received a transition period of four years, and Lithuania received a transition period of six years.
The agreement also stated a period of transition for the Baltic countries to last until December 31, 1998+1
1998+1 is 1999, the year mentioned in the statement. I think the model got confused because of the math.
( z1 state :ARG0 ( z2 agreement ) :ARG1 ( z3 transition :ARG1 ( z4 country :location ( z5 world-region :name ( z6 name :op "Baltic" ) ) ) :duration ( z7 date-interval :op ( z8 until :op ( z9 date-entity :year 1998 :month 12 :day 31 ) :op ( z10 date-entity :year 1998 :month 12 :day 1 ) ) ) ) )