@prompt condition

Hi Guys,

I Have an object to write a @prompt condition with Values to range from last completed month to 24 months prior (current month -1 to current month – 25).

for example:

             If current month is January 2016, then values tor range contiguously from December 2015 – January 2014

Here is the  conditional requirement :

Add a value of “Default” to the top of the list of values. Date values to be sorted in Descending order. All contiguous Month Year values to be present in the list of values up until current month – 25.

Prompt is to be mono-select. User can select either 1 Month Year value or the value “Default”.

Default = previous month – 6 months

* When refreshing a report, if the value Default is selected for both the Period Begin and Period End dates, the report will return 6 months worth of data

Please Suggest the syntax or any work around's for the above condition

Thanks in advance,

Thanks,

Sankar.