Once we take into account that the basic text of the Diary was drafted in 1991 (because 1990 is a fairly obvious typo)
Contrary to popular Diary Defender belief I don't just make things up.
I have seen a copy of Mike's affidavit with "1990" corrected to "1991" throughout, along with other obvious typos. I don't know who did it - possibly Alan Gray although I really can't say - but it would be odd if it was an unauthorized correction (because how would the person making the corrections have known that 1990 was a typo?) and that is what leads me to think that 1990 was indeed a typo and was intended to be 1991 (even though I obviously think that should have been 1992).