For at least the last 20 years, probably longer, the dates of the TT have always been set by the middle (Mad) Sunday. This is always the first Sunday in June. This year the first Sunday in June is the 7th, so the TT is as late as it can possibly be. Last year Mad Sunday was on 1st June, so the TT dates were the earliest they could be.

Having said that, I'm pretty sure that years ago the TT was a week or so later than it is now. I seem to recall Senior Race Day being as late as 16th June or thereabouts.