Want to add a "variable" to a table
Timestamping each row doesn't seem to make a lot of sense to me. I would just keep a log of the events that are interesting. Such a log could be reusable for other events too. E.g. (sorry this is SQL Server dialect, you may have to adjust slightly):

CREATE TABLE dbo.EventLog
(
  EventDate SMALLDATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
  [Description] NVARCHAR(2000)
);


Whenever you reload the table, also call a stored procedure that does this:

INSERT dbo.EventLog([Description])
  SELECT N'Re-populated table x';


If you need to know the date of the last refresh, you can say:

SELECT MAX(EventDate) FROM dbo.EventLog
  WHERE [Description] = N'Re-populated table x';


If all you ever care about is the last time, then before the insert above, you could just:

DELETE dbo.EventLog
  WHERE [Description] = N'Re-populated table x';