The details of the war are vague. The description on the official site says that the war has ended. The exact time frame is unknown, although it likely ended during the 1980's as the only confirmed veteran of the war is still a young man in 1982. At the beginning of the game, while it is implied that the border town of Grestin has been divided into two and has remained so for the past six years, this does not provide exact dates of the war – it could mean that the situation developed at some point before, during, or after the war.
After the war, the relationship between the two countries is still somewhat hostile, hence the series of attacks caused by Kolechian terrorists.
If Sergiu is alive at the beginning of day 22, he will tell the inspector that he fought in the war on the Arstotzkan side for five years. Having just survived two terrorist attacks (days 18 and 21), he also mentions that "there is more action on this checkpoint than in the war", therefore implying the Six-Year-War was either fought with little intensity, or Volda was not involved in much action, although this could also simply be him pointing out the absurd amount of attacks the border suffers.