I found another source that said she was 94 and died in 1986.
And then, Lomax says it was September 1985. 
Anyway, it was after the murders and before the trial.
The most relaible information I could find is she died in February 1986.
Indeed how could she have asked about Jeremy in December 1985 if she was dead?
It is not unheard of for people to say he lost the race by mere seconds when referring to 600 seconds instead of saying 10 minutes. Technically it is true but it gives a rather false impression.
He could have been engaging in that kind of behavior. Since I find much of his work to be sloppy and questionable he also could simply have been wrong and did a pisspoor job of researching the issue.
EIther Lomax is purposely calling months days to try to make it sound like she died very quickly after she changed her will or he was alleging she died in September 1985 but flat out wrong.