https://www.dotnetzone.gr:443/cs/blogs/birbilis/archive/2012/07/14/fix-the-tag-timeupdown-does-not-exist-in-xml-namespace-silverlight-toolkit.aspxIf you use TimeUpDown control from Silverlight Toolkit in your XAML like below (copy pasting from CaptionGrid at ClipFlair source): <ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"elCommunityServer 2.1 SP3 (Build: 20423.1)