before adding a new metafield (as Georgi has explained), you can try to edit the ~/Sitefinity/Admin/ControlTemplates/News/ControlPanelInsert.ascx and ~/Sitefinity/Admin/ControlTemplates/News/ControlPanelEdit.ascx templates and replace the RadDatePicker control with ID of "Publication_Date" to RadDateTimePicker control (leave all the properties intact, only the control should be changed).
The actual problem is that our UI is sending only the information about date, but not the time. However, the code and the database are well equipped to work with time as well. So if the user interface sends the time information as well (as I have described in first paragraph), you should be able to publish news on the particular time.
Let us know if you have any additional questions.
All the best,
the Telerik team