It depends. If the event did in fact happen in another rating period (and nothing new has surfaced regarding this event since then), it probably shouldn’t be mentioned in the current report.
However, old situations can be referenced if there are new related developments or offenses. For example:
A Soldier was picked up for DUI and found guilty. The initial investigation revealed that the Soldier was above the legal limit by use of a blood alcohol test. His evaluation report could mention the facts regarding the DUI. If the final report comes back during the following rating period and it indicates that the Soldier also tested positive for cocaine at the time of the DUI, the situation would likely be mentioned in that NCOER as well.
In this case, since there is new information regarding a previous event that was only discovered during the following rating period, then the conduct can be referenced.
