Help Modifying a custom report to give me the ability to enter specific dates (pull historical) and then default to the last "Month"

In the web reporting you can use the ${FromTime} and ${ToTime} values. [Explained in more details on Using SWQL/SQL with web-based reports (solarwinds.com)]

Your first report query would change to this: [UPDATED]

SELECT [Nodes].CustomProperties.AvailabilityReportingGroups as [Team]
     , [Nodes].Caption as [Node Name]
     , CONCAT(ROUND(AVG( [Nodes].ResponseTimeHistory.Availability ), 2), '%') as Avail
FROM Orion.Nodes AS [Nodes]
-- Old WHERE clause
--WHERE [Nodes].ResponseTimeHistory.DateTime>ADDDAY(-30, GETDATE())
-- New WHERE clause (my preferred way)
WHERE [Nodes].ResponseTimeHistory.DateTime BETWEEN ${FromTime} AND ${ToTime}
-- Other way
-- WHERE [Nodes].ResponseTimeHistory.DateTime >= ${FromTime}
--   AND [Nodes].ResponseTimeHistory.DateTime < ${ToTime}
GROUP BY [Nodes].CustomProperties.AvailabilityReportingGroups
       , [Nodes].Caption
ORDER BY [Nodes].CustomProperties.AvailabilityReportingGroups
       , [Nodes].Caption DESC

Note: I changed a few things to get in line with the way that I prefer my queries, but functionally, it's the same.

Same note applies to your second query. [UPDATED]

SELECT [NodeCPs].AvailabilityReportingGroups as [Team]
     , CONCAT(ROUND(AVG( [NodeCPs].Node.ResponseTimeHistory.Availability ), 2 ), '%') as [Availabilty]
FROM Orion.NodesCustomProperties [NodeCPs]
--Old WHERE clause
--WHERE [NodeCPs].AvailabilityReportingGroups IS NOT NULL 
--  AND [NodeCPs].Node.ResponseTimeHistory.DateTime>ADDDAY(-30, GETDATE())
-- New WHERE clause (my preferred way)
WHERE ISNULL([NodeCPs].AvailabilityReportingGroups, '') <> ''
  AND [NodeCPs].Node.ResponseTimeHistory.DateTime BETWEEN ${FromTime} AND ${ToTime}
-- Other way
-- WHERE ISNULL([NodeCPs].AvailabilityReportingGroups, '') <> ''
--   AND [NodeCPs].Node.ResponseTimeHistory.DateTime >= ${FromTime}
--   AND [NodeCPs].Node.ResponseTimeHistory.DateTime < ${ToTime}
GROUP BY [NodeCPs].AvailabilityReportingGroups

When pasting into my report i simply get "Query is not valid".  

Also to note the option to select a time frame is gone, so I assume i have to hard code it somehow?

I might have a typo because I don't have that particular Custom Property in my system.  Give me a few minutes to add it and I'll test the queries.

I might have a typo because I don't have that particular Custom Property in my system.  Give me a few minutes to add it and I'll test the queries.

Yep - one too many closing parenthesis in the custom query.  I've updated the above ones.

Okay, good news it likes your queries now.  But on to the second part of the question.  Even with the $FromTime and $ToTime - How do I declare these variables?  I dont see the selector like I would with a normal query?