Finding month and month start and end date from start and end date.

The dateDiff() function can tell you the number of months between the two dates.  Then you would use a loop to increment over each month and use the DaysInMonth() function to tell you the end date of each month.  The start date of the month is easy.. it is always the first!

No custom tag.. but there is custom code.

<cfset startdate = '11-01-2009' />
<cfset endDate = '10-31-2010' />

<cfset numberOfMonths = dateDiff( "m", startDate, endDate ) />

<cfloop from="0" to="#numberOfMonths#" index="idx">
    <cfset loopDate = DateAdd( "m", idx, startDate )>
 
  <cfset beginningOfMonth = CreateDate( year(loopDate), month(loopDate), 1 )/>
  <cfset endingOfMonth = CreateDate( year(loopDate), month(loopDate), daysInMonth(loopDate) )/>

    <cfoutput>
      #dateFormat(beginningOfMonth)# to #dateFormat(endingOfMonth)#<br/>
  </cfoutput>

</cfloop>