Student or Learner
On 9/11 the sky has been the clearest for 10 years.
OR with the comma
On 9/11, the sky has been the clearest for the past 10 years.
For the past ten years, the sky has been the clearest on 9/11.
What I would like to know is whether any of the above is grammatically correct and if it's confusing or not.
What the person was trying to say is that on that particular date, the sky has been the clearest (less pollution, regardless whether it's a fact or not) within a 10 year period. Meaning that the air was more polluted before and after that time, and on that day only, the sky had the least pollution. For some reason this does not make a lot of sense to me.
Any suggestions as to what it should be like. All help is appreciated: THANKS!
In all three sentences, you've used the present perfect, which implies a connection between the past and the present. It means, in fact, that today is 9/11, and the sky is clearer today than it ever has been in the last ten years.
But you're referring to a specific date in the past, and for that we use the past simple tense: "On 9/11, the sky was the clearest it has been in the last ten years." The sky was clear then (past simple), and we are talking about a time period that began 10 years ago and ends in the present (connection between past and present -> present perfect).
The comma makes no difference to the meaning, but it is probably good style to include it here. When you reverse the order of your clauses -- "For the past ten years, the sky has been the clearest on 9/11" -- this implies that for at least the last ten years, on every 9th September, the sky is clearer than it is on any other day.