Start recording a macro. In the "Record Macro" dialog, change the macro name
to "Date_Time" (without the quotes), make the shortcut key CTRL+T (capital
T), make sure it's stored in This Workbook, click OK.

Stop Recording.

Press ALT+F8, highlight Date_Time in the listbox, click Edit. Replace
everything between the "Sub" and "End Sub" lines with

ActiveCell.Value = Now

Press ALT+Q to quit the VBE. Back in the worksheet, use the CTRL(+Shift)+T
shortcut to call it.

HTH,
Andy