Use after update to change dates

In an Access form there are two text boxes(StDate, EndDate) that represent a date range. The default property is set at Date()-1 for StDate and Date() for EndDate. There are two more text boxes(StDate2, EndDate2) that are also a date range. StDate2 and EndDate2 are dependent on StDate and EndDate. All of these can be initially set in the text box porperties. However if the user changes the StDate I would like to use the After Update event to change the second date range accordingly. What code would be necessary to accomplish this ? StDate2 and EndDate2 are 1 day less than stDate and EndDate.  Thanks.