Hi Luigui,
First, set your date format to show you time as well as dates. (Tool
->Options, View tab, set the Date format to show time.) Project calculates
information to the minute, so viewing time as well as the date can frequently
help track down problems.
On the Calendar tab in the Options dialog box, correct the default start and
end times as well as modify the number of hours in a day and a week. It is
this definition of "days" and "weeks" that Project uses when you enter task
durations. So if your project calendar was modified for a 7.5 hour work day,
but the definition of a "day" was 8 hours, the task will not end until 1/2 on
an hour into the next working day.
You may also want to check the start time of your Project (Project ->
Project Information) to verify that the project is starting at the correct
time.
Hope this helps. Let us know how you get along.
Julie