I've read somewhere that there is undocumented parameter 'TZ' in flat protocol which allows you to specify time zone. I've tried to add it to my XML-RPC request, but it does not work.
To me it looks like very serious protocol deficiency. It asks to provide information from user profile which I have to way to retrieve. It would be stupid for my to ask user for his time zone in UI each time. Are there any workarounds? What other client developers do? Nobody uses XML-RPC protocol anymore?