Remember Project stores everything in minutes. The "Hours per Day" setting
is the conversion factor that controls how an entry of, say, "3 days" in the
duration field is converted to minutes for storing and calculations. If you
want a "day" to be considered a 24 hour period of time, that's where you set
it. But be aware that it is a global setting covering all tasks.
Be careful with the 24 hour calendar, it can bite you in the behind if
you're not. If you have a task that lasts 3 24-hour days, that's 72
consecutive hours. That means once work on it starts it will continue
without interruption for 72 hours. Machines can work like that but people
need meal and sleep breaks. Since tasks should be broken down to the level
of one task = one resource, using the 24 hour calendar means your employees
are going to be expected to work for days upon days on end without any
breaks or time off and that just doesn't describe most normal work
situations. 3 8-hour calendars, one for each shift, means a worker might
work on a task during the day and then be relieved by another for the
evening shift, workers coming and going but the task going round the clock
until it is done. In that case 24 hours does represent 3 8-hour workdays
which for that task just happen to happen consecutively within the same
calendar day. But the 24 hour calendar implies that ONE guy or a team
working together at the same time as a unit is going to work continuously on
the task from start to finish without a break, even if it takes days or
weeks to complete the task.
