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Login to steam and download the left 4 dead 2 demo. (This should create the client.dll for you)
Once left 4 dead 2 demo is at 1% pause the download. Navigate to C:...\Steam\steamapps\common\left 4 dead 2 demo\left4dead2\bin and copy the Client.dll
Replace Client.dll from l4d2 demo into your installed l4d2 folder i.e. X:...\Left4dead2\Left4dead2\bin
I didn't change the date or the timezone and it works.