Auto populate week commencing date

Assuming your other date field is called "Date", enter the following custom calculation script in the field you want to populate from it's value:

if(!this.getField("Date").value)
{event.value=""}
else
{
var date=util.scand("dddd, dd mmm yyyy",this.getField("Date").value);
var day=date.getDay();
var diff=0;
if(day==0){diff=1}
else if(day>1)
{diff=1-day}

date.setDate(date.getDate()+ diff);
event.value = util.printd("dddd, dd mmm yyyy", date);
}