Handling strings and dates, as well as calculations, is a pain in SharePoint Designer workflows. The links mentioned in Colin are good, but you can also consider embedding C # or vb.net code directly in the SharePoint Designer workflow (without the need for Visual Studio).
I wrote a component that allows you to do this and blog about it at the following URL:
http://www.muhimbi.com/blog/2009/12/embed-c-or-vb-code-directly-in.html
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