Work on tasks only takes place during the hours that the calendar says are
working hours. If your workday is 8-5, a task cannot start at 6am because
there won't be anyone there to do it. Setting aside the idea that with rare
exceptions one should never be entering start or finish times anyway - 99%
of the time those should be calculated by Project, doing those sorts of
calculations is why the software even exists in the first place - you can
enter a task starting at, say, 6am on Oct 19th, just by entering that date
and time in the Start column. (All date fields in MSP are really date/time
fields, whether you've chosen to display the times or not.) Making that
entry will in fact create a Start No Earlier Than constraint of 6am on that
date. But since working hours don't start until 8am, Project moves the
scheduled start to 2 hours later, 8am, the earliest time anyone is actually
going to show up to do the work. If the person doing the task actually
works 6am to 3pm instead of 8am to 5pm and you have adjusted his resource
calendar to reflect his correct hours as you should, when you assign the
resource to the task its start will move back to 6am as you want it to. You
can also get it to 6am by creating a base calendar that shows hours of work
starting at or including 6am and setting it as the task calendar for that
particular activity.
