Thanks. I have the same idea but I am stuck. Probably eventually I will find the script which takes date in 2009-03-31 format and determines the week. Right now we put it manually
BTW week in CalendarDateField selector is shown differently than date('W') returns (probably because in NZ it is counted differently).
However there is another, more deep question. Assume that I want to store in database some function of of other fields: f.e. I have lastname, firstname and fullname which should be "firstname lastname". How to create CustomPage.php which creates an entry form for firstname and lastname but enters fullname automatically (this example is obviously artificial).