Date Expression - calculate a month

I have two parameters for date, @Report_From_Date and @Report_To_Date. I want to set their default values to latest Month. So for my @Report_From_Date default value I have [quote]=DateSerial(Year(Now()), Month(Now()), "1")[/quote] and @Report_To_Date I have [quote]=DateSerial(Year(Now), Month(Now), "1")[/quote]. this as a results it gives me the 1st of October, but I want to get the 1st of every month to the last day of that month. help.