There is a common misunderstanding what those default start and end times
actually are. First, they are NOT the 0800-1200 and 1300-1700 default work
hours you find in the working time calendar, the work schedule you get if
you click the "use default" radio button in the upper right corner of the
Change Working Time calendar window and changing them does not change that
behavior. Those work hour defaults are hard coded and cannot be changed,
not even by a registry hack or some other back door.
The default times you've changed are something else entirely. All of the
dates in Project are actually date/time fields. When you enter a start date
using the little pull down calendars, like when you set the Project Start
Date or set a constraint or deadline or click in the Start field in the task
list to set a start date, Project uses the working time calendar to supply
the time portion of the entry. But when you actually type in the date
string manually, Project will use the default start you've designated to
fill the time portion of the entry. Same for entering finish dates.
Here's a little experiment to demonstrate it. Set up a little project with
three tasks, Project Start date (Project, Project Information menu) on
Monday. Leave the calendar as the normal 0800-1200 and 1300-1700 and use
the options screen to set default start to 10:00. Do NOT click "set as
default" unless you want these settings to apply to all projects. In the
Tools Options View page select a date format that shows you the time as well
as the date. For the first task in your project just enter the name and hit
enter - don't change anything else. For the second, click in the start
field and from the little calendar pulldown set a start date (ie,
constraint) of next Wednesday, 27 Oct. In the start field of the third task
TYPE "10/27/2004" without the quotes of course (assuming you use US format
mm/dd/yyyy dates) and hit enter. You'll see task 1 shows starting at Monday
8am, task 2 will show Wednesday at 8am and task 3 will show Wednesday 10am.
That task 3 time is what those default start and end times control.