Bacth job to run monthly

hello world

My batch runs everyday at 00:00 and prints a report, put some of the data into a table.

Now I am requested to change it to run monthly, not everyday.

How can I change the following to do the same but not every midnight, but one midnight monthly.  If possible I'd like it to run on a certain date for each and every month for consistency reasons. e.g. batch runs on the 28th of every month at 00:00 and print the report.

public void PurchTabletoBatchTable()
{
    PurchReqTable PurchReq;
    Axn_PurchCommitmentAccTable purchCommitmentAccTable;
    date Tday;
    utcDateTime utcTday, utcFromDate, utcToDate;
    int timeOfday;
    createdDateTime PurchReqcreatedDateTime;
    ;
    Tday = today(); //get today
    timeOfday = str2Time("00:00"); //prepare time
    utcFromDate = datetimeutil::newDateTime(Tday,timeOfday); //convert to utcDateTym

    ttsbegin;

    while select PurchReq where PurchReq.status == PurchReqStatus::PendingApproval && PurchReq.createdDateTime >= utcFromDate &&
    PurchReq.createdDateTime < utcToDate

     {
        purchCommitmentAccTable.PurchReqId = PurchReq.PurchReqId;
        purchCommitmentAccTable.Status = PurchReq.Status;
        purchCommitmentAccTable.Requisitioner = PurchReq.Requisitioner;
        purchCommitmentAccTable.PurchReqcreatedDateTime = PurchReq.createdDateTime;
        purchCommitmentAccTable.insert();
     }
    ttscommit;

}

Please help a girl out.

Thanks

Sinoyolo