The formulas you've suggested are almost perfect.

1) A1: Enter the date to use for this sheet. I do not recommend =TODAY() as that will change everytime you open the workbook, not good for historical sheets.

2) To show date from A1 plus 8am in another cell:

=A1+TIMEVALUE("8:00 AM")

3) Be sure to apply a number format to that second cell that will display the date AND the time... such as mm/dd/yyyy h:mm