time frame time to sepcify.

i have a search where user selects the timeframe and based on that i peform the serach,
example:
if they select past day from the dropdown i pass DateTime.AddDays(-1).ToString() and pass this to my query.
if the select past month from the dropdown i pass DateTime.AddMonths(-1).ToString() and so on..
I also Have a ddl item as AnyTimeFrame - so I wanted to know what should i pass when they select this option.
Thanks

...