>>>the following code in a SQL interrogation always return "0-12"
>>>
>>>

>>>IIF( (HOUR( TTOD( Table1.datecreation)) >=0  AND HOUR( TTOD( Table1.datecreation)) <= 11), "0-12", IIF( (HOUR( TTOD( Table1.datecreation)) >= 12 AND HOUR( TTOD( Table1.datecreation)) <= 15), "12-16", "16-24")) as HourBlock
>>>

>>>
>>>Data contains hours in all the range 0-24. So I should have some "12-16" and "16-24"
>>>
>>>You guys see something that I don't?
>>
>>What's Set('hour')? Hint: it should be 24 in your case.
>
>I thought about it but the help says that "set hours" doesn't make a difference.

I don't see that in help, may I ask where you see it? Anyway, I would try Set Hour = 24 and see what happens to your query.