Update a table's responses' datetimes so that all records have occurred recently

update-datetime.sql

-- @last = the very last response
SELECT @last:=UNIX_TIMESTAMP(response_date) FROM responses GROUP BY response_date DESC LIMIT 1;

-- @diff = how much time has passed since the last response
SELECT @diff:=(UNIX_TIMESTAMP() - @last);

-- Add the date difference to all responses to chronologically bring them up to date
UPDATE responses SET response_date=FROM_UNIXTIME(UNIX_TIMESTAMP(response_date) + @diff);

Assumes the name of the table is responses and the response_date is of DATETIME type.