Not able to decipher the encoded date parameters

On selecting a from and to date the from date and to date are encrypted and sent to the councils as parameters. We are trying to decipher this encoding , so that we can send the proper encoded from and to date as dynamic parameters. Can you please help us to do so.
For your reference the URL is,

http://www.ashford.gov.uk/online_planning/DCCore/default.aspx?newsearch=true

Go to planning applications << Select the Date tab

On Viewstate and Eventvalidation value gets changed

Searched with Apr dates. Its worked but when run for different month its get scraped for only 1st date of that particular month. The value of Viewstate and Eventvalidation gets changed for every selection.
For example:
Captured with April dates now changed the dates with 01 mar to 05 mar, collecting only 01 mar to 01 mar

It looks like when you click

It looks like when you click a date, it sends off an HTTP request, and the encoding is in the response. So you would want a scrapeable file to select start date, maybe another for end date (or you may be able to reuse the previous) then a last request that submits the whole from with those parameters.