Parameter for Current Month and Previous Month

Question

I'm trying to create a parameter for current month and previous month based on the ex_date, but not sure what i'm doing wrong. 
where ex_date = @SelectDate
I created a second dataset below for the values in the parameters.
SELECT Month(CURRENT_TIMESTAMP) AS 'Month', 'Current Month' as 'Current Month'
union all
SELECT Month(CURRENT_TIMESTAMP)-1 AS Month, 'Previous Month' as 'Previous Month'
Results
Month Current Month
3 Current Month
2 Previous Month
Once I preview it I get "Conversion failed when converting date and/ or time from character string" I changed the data type to "date/Time" but that did not make a difference. The date is convert (varchar(10), ex_date, 101) so looks like
11/12/2014. 
I've also tried expressions like =month(now()) to pull current month with same error so i'm not sure what i'm doing wrong. Any ideas?

Answer 1

i tired this real simple report
 in the first dataset - my main report query - select name from sysdatabases where month(crdate())=@month
in the second dataset - select month(getdate()) as Month1
in the parameters - choose int data type and available values - select the second data set
in the first data set- add this parameter..( i am assumming you know this, since you have done)
in the preview you should get the drop down with current month number - 3
and if you run the report, it will display the database names that were created in march. remember we are no checking year, so will get all that were created in march across the years.
Hope it Helps!!
I'm looking to have the dropdown say "Previous Month" and "Current Month" as a option. I know how to get the information in SQL, but not sure how this translates or put into a parameter.
Current Month
list_date BETWEEN
DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()), 0)
AND
DATEADD(MONTH, DATEDIFF(MONTH, 0, GETDATE()) + 1, 0)
Previous Month
list_date between
CONVERT(varchar,dateadd(d,-(day(dateadd(m,-1,getdate()-2))),dateadd(m,-1,getdate()-1)),106) /* Last Month */
and
CONVERT(varchar,dateadd(d,-(day(getdate())),getdate()),106)