Set Initial Value Of Subsequent Date Fields Based On First

I have an HTML form which contains two date fields to accept airport taxi bookings, sometimes clients book one way, sometimes they book return journeys, some cases even involve 3 or more journeys.

Now there is a field for the date of each journey all contained within one HTML form:

date0, date1, date2 etc... depending on how many legs there are to their journey.

What I'd like to do is set an initial value for all date fields based on the value the user enters into the first.

So if the user enters: 24/04/2009 into [date0] field then [date1] should be automatically set to be 7 days later and [date2] 7 days after [date1] etc.

The user must be able to overwrite the initial values, without affecting the previous dates.

Code:
<form name="form1" method="get" action="booking-function.asp">
<%'ASP Loop Condition for number of date fields to display%>
<input name="date<%=(x-1)%>" type="text" maxlength="10" />
<%'End ASP loop%>

[Code].....