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Add the Due date column (View->Columns->Due Date), then you can just tab along the row and type in the dates/times. Also, entering dates in the row allows you to use various shorthand notations, like "+1d" (1 day from today), "Fri" (Friday, use the 1st 3 letters of the day name only), "Tue +2w 4p" (two weeks from this coming Tuesday, at 4 pm), etc. See the help for "Setting start dates and due dates" in OmniFocus for more details.