If you're using the Date & Time button on the Insert ribbon, uncheck the "Update automatically" box in the lower right of the dialog. The date will then be inserted as plain text that won't update.
If you use the Alt+Shift+D shortcut, or if you insert a Date field through the Quick Parts > Field dialog, you can change it to plain text by clicking on it and pressing Ctrl+Shift+F9 (the shortcut of the Unlink command).
If you want the date to be the day the document was created, insert a CreateDate field instead of a Date field. The good tip is that you can insert a CreateDate field at any later time and it will still show the date the document was created, not the date the
field was inserted.