Crystal Reports Calculating first Monday date in week

Message

Hi I am trying to create a date range for a weekly enrolment report . At the minute I am trying retrieve the Monday date of the week the student enrolled based on there enrolment date  . 

At the moment I can only get the last Monday in that Month .

Can anybody help please.

Local DateTimeVar d1 := {stmaos.dt_achieved};
Local DateVar d2;
d2 := DateSerial(Year(d1), Month(d1)+ 1, 1 - 1);
d2 - (DayofWeek(d2, crMonday)-1)