It can be a bit confusing so the following example should help clear up how policies over a certain timeframe will be evaluated.  A sample policy condition is the following:

Total Cost ($) increased by more than 20 % over 7 days.

In this example, CloudHealth will evaluate between the last full day of evaluation which is generally 2 days prior to the current date (day X) and 7 days before (day Y).  We then check if there was an increase of 20% or more from day Y to day X.

For more information regarding policy configuration, see:  Best Practice Policies