The Incident:

Barry Castillio, Lazar's coworker, told the story of an altercation that took place in 1979 between the aliens and the humans at the base.  Apparently, the aliens were given a location at the facility to conduct experiments.  It was agreed between the aliens and humans that there would be human military guards  securing the immediate perimeter.  One day, due to the volatility concerns the aliens had over a certain experiment that they were conducting, they demanded that the guards not be present.  The concern was that the bullets in their guns and gunbelts would cause a reaction.  The guards refused and proceeded to come closer to the danger area.  They were immediately terminated by the aliens, later found by two other guards to be lying dead of identical head wounds.

The actions and reactions of these events escalated until 44 military guards  were killed.  The aliens then killed the scientist they were teaching and left the facility and much of the alien technology behind.

The back engineering program began in late 1979 with the remaining hardware and technology.  Before the aliens left, they somehow stated that they would return at a date represented by six digits starting with 1625xx.    Lazar was given no cross reference for this date to our numbering system and couldn't speculate as to what the date meant.  The xx's indicate unknown numbers.