[plt-scheme] how to work with dates in scheme

Hi,

could anyone please give me a starting point how to work with dates in
scheme (apart from the PLT scheme reference, where I did not find what I
was looking for)?

For example, I need to get a date as a program argument in some form
(e.g., a string like 01.01.2009) and query the database for entries for
that day and some following days.

Thanks a lot for any help,
Sigrid