LastModified in CQL not working

I recently started to study the XML RPC API in order to use it in a client. When I come to the method "search", I tried different interesting CQL queries. The problem is, the field search "LastModified" does not seem to work. No matter which date and format I specify for searching (e.g. LastModified >= 2010-01-01, LastModified>="2010-01-01" etc.), I do not get any result, although I am sure there must be some pages older than that date.

This is somehow critical to my use case, since I would need to get the changed/modified pages, not all of them every time.