You can use many different formats. Your OS settings will define what's acceptable, e.g. on Win 7 UK edition Windows here are just some of the valid formats:
-ma "20 june 2012"
Since what's acceptable can vary according to machine I recommend using your reverse date format if you want to use the command line on different machines, e.g.
If you want to load settings from a SRF file AND provide values at the command prompt you need to also include the -po switch, e.g.
agentransack.exe MySearch.srf -po -ma "2012-06-20"