(1) Explain your problem, don't simply post "This isn't working". What were you doing when you faced the problem? What have you tried to resolve - did you look for a solution using "Search" ? Has it happened just once or several times?
(2) It's also good to get feedback when a solution is found, return to the original post to explain how it was resolved so that more people can also use the results.
Hi Abbas,
The recommended best practise is the DD is always the last working hours of the cut-off date on calendar assigned on the project or if working per shift, the data date should be last reporting hours of the reporting shifts and not the start otherwise your histogram or bar will have combination of actual & forecast specially when using a multiple calendar.
For you question of maintaining the hours, all you need to do is activate the hours (Edit->User Preference->Dates then select 12 hour) before changing the data date when running the schedule.
If you are using P6 rel 21, this issue is now fixed. The hours is shown and maintain so you don't need to change on user preference.