Sample code for an onBeforeGet server event to replace the string "yesterday" in a date property with code that will search on yesterday's date.

SearchOnYesterday.cs

Innovator inn = this.getInnovator();
string createdOn = this.getProperty("created_on", "");
if (createdOn == "yesterday")
{
    DateTime yesterday = DateTime.Today.AddDays(-1);
    DateTime eodYesterday = yesterday.AddHours(23).AddMinutes(59).AddSeconds(59);
    this.setPropertyAttribute("created_on", "condition", "between");
    this.setProperty("modified_on", String.Format("{0} and {1}",
                        yesterday.ToString("yyyy-MM-ddTHH:mm:ss"),
                        eodYesterday.ToString("yyyy-MM-ddTHH:mm:ss")));
    return inn.newError(this.node.OuterXml);
}
return this;