Query Results and my recent hair loss

Question

I'm pulling my hair out trying to solve this problem.
I have a single table of data, tblVisitLog;
dos - datetime
location - nvarchar
ticket
pat_num
pat_name
insurance
proc_code
proc_name
mod_code
diag_cod
diag_name
charge - money
groupid - int
First I build a 2-year look back with this query for a chart, and generate a 1 month look back if they haven't selected a DateRange.
I'm finding that there is something wrong with my query that is produced by the user. The number of visits (count of distinct ticket) and the related charges are wrong. It seems to be off by only a few visits, and my first thought was that it had something to do with the time portion of the datetime data, but all my times are 12:00 AM in the DB.
I'm a bit new at CFM, and rusty at best on SQL, so if anyone can provide some guidance, I'd surely appreciate it.
<!--- Get 2 past years data --->
<cfquery name="ChargesByDosQry"  datasource="myDatabase" username="non" password="non">
DECLARE @FirstDate datetime
DECLARE @LastDate datetime
SET @FirstDate = DATEADD(mm, -1, DATEADD(yyyy, -2, CONVERT(datetime, CONVERT(char(7), getdate(), 120) + '-01')))
SET @LastDate = CONVERT(datetime, CONVERT(char(7), getdate(), 120) + '-01')
SELECT month(dos) as MonthNo, year(dos) as YearNo, COUNT(DISTINCT Ticket) AS Visits, SUM(charge) AS Charges FROM tblVisitLog
WHERE dos >= @FirstDate AND dos < @LastDate AND (groupid = '#GroupID#')
GROUP BY year(dos),month(dos) ORDER BY YearNo, MonthNo
</cfquery>
<cfif IsDefined("DateRange")>
     <!--- Get date range data if the user selects a date range --->
          <cfset sep="#Find("to", Form.DateRange)#">
          <cfset StartDate="#Left(Form.DateRange, sep-1)#">
          <cfset EndDate="#Right(Form.DateRange, sep-2)#">
          <cfquery name="MonthQuery" datasource="myDatabase" username="non" password="non">
             SELECT *
             FROM tblVisitLog
             WHERE dos >= '#StartDate#' AND dos < DATEADD(Day, 1, '#EndDate#') AND (groupid = '#GroupID#')
          </cfquery>
<cfelse> <!--- Get last months data if no date range is selected --->
          <cfquery name="MonthQuery" datasource="myDatabase" username="non" password="non">
           DECLARE @FirstDayOfMonth datetime
           DECLARE @FirstDayOfLastMonth datetime
           SET @FirstDayOfMonth = CONVERT(datetime, CONVERT(char(7), getdate(), 120) + '-01')
           SET @FirstDayOfLastMonth = dateadd(mm, - 1, CONVERT(datetime, CONVERT(char(7), getdate(), 120) + '-01'))    SELECT *
           FROM tblVisitLog
           WHERE dos >= @FirstDayOfLastMonth AND dos < @FirstDayOfMonth AND (groupid = '#GroupID#')
           </cfquery>
</cfif>
        <!--- Get census for months data   --->
         <cfquery name="CensusQry" dbtype="Query">
              SELECT COUNT(DISTINCT ticket) AS CensusTotal
              FROM MonthQuery
         </cfquery>
        <!--- Get total Charges for months data    --->
        <cfquery name="ChargesQry" dbtype="Query">
          SELECT SUM(charge) AS TotalCharges
          FROM MonthQuery
        </cfquery>
        <!--- Get unique insurance info --->
        <cfquery name="InsuranceQry" dbtype="Query">
          SELECT insurance, COUNT(insurance) AS InsCount, SUM(charge) AS Charges
          FROM MonthQuery
          GROUP BY insurance
        </cfquery>
        <!--- Get distinct location names and count --->
        <cfquery name="SiteQry" dbtype="Query">
        SELECT location, COUNT(location) AS LocCount, SUM(charge) AS Charges
        FROM MonthQuery
        GROUP BY location
        </cfquery>

Answer 1

Seems like some overengineering taking place in your first query.  Instead of all those declarations and converts, why not a simple.
TwoYearsAgo = DateAdd("yyyy", -2, now();
where dos >= <cfqueryparam cfsqltype="cf_sql_date" value = "#TwoYearsAgo#">
and dos < getdate()
cfsqltype="cf_sql_date" strips out the time portion for you
When processing user input, you are using functions that produce strings and then use those strings as dates in your query.  That might be messing you up.
Why do you think you are off by a couple of visits?  Do you run the same query outside cf and get different answers?