Depending on if you need the time component, it may just be eaiser to
work with the Date object.

d = Date.today
lw = d - 7

'lw' will be 7 days ago.  In this case adding and subtracting seconds
will be fine, but be careful about it, days aren't always 24 hours long
(think about daylight savings time), and it can really catch up with
you sometimes.

  .adam