Fill in the Since field with a time value or keyword from
  which you want to display history information. CMS
  displays history information on or after the time you
  specify.

  Time values can be an absolute, delta, or a combination
  time value.  For example, 17:12:37 is a valid absolute
  time value.  Keywords can be one of the following:
  TODAY, TOMORROW, or YESTERDAY.

  If you do not enter a value, CMS displays history records
  for the past 30 days.