I found the answer. Here it is:
// Find the first date of the month based on the date field
datetimevar firstDate:= datetime(year({DC_STOCK_OUT_LOW.cal_date}), month({DC_STOCK_OUT_LOW.cal_date}),1,0,0,0) ;
// Find the first date of the next month based on the date field
datetimevar nextMonth:= dateadd("m",+1,firstDate);
// Subtract one day from the first date of the next month. This gives you
// the last day of the month based on the date field.
nextMonth:=dateadd("d",-1, nextMonth);
//day(nextmonth)
// Finds the Percentage of stock out by dividing the total stock outs by the days in the month
{DC_STOCK_OUT_LOW.STOCK_OUT_DAY}/day(nextMonth)