There's a nice way around this.
Early in SS, Kirk mistook Khan's DY spaceship for a much later version. Even through he was corrected by Spock, he continued to think of the ship as be from the era that constructed DY500s'. Kirk would often place his own perceptions ahead of Spock's (sometimes rightly).
When Khan asked Kirk "how long," Kirk told him about two centuries. Again based on Kirk earlier misconception of when Khan's ship was from.
Fifteen years later, Khan was still operating off the (mistaken) notion that he was about two centuries in the future. We have no idea just how quickly Marla was killed and from where else would Khan have learn of the correct date?
That is frickin' genius, and I buy it completely.
I'm a 300-years-from-NBC-broadcast guy and I like the cut of your jib!