With the Q1 2016 release of UI for WPF, Telerik introduced the Today button to provide the ability to select the current date and time of RadDateTimePicker. By default the button is not visible. However, you can make it visible by setting the TodayButtonVisibility to Visible. You are also able to change the current content of the button using the TodayButtonContent.
When the InputMode is set to DatePicker and the user presses the Today button, only the current date will be selected. When the InputMode is set to TimePicker and the user presses the Today button, the current time will be selected.
[XAML] TodayButtonVisibility and TodayButtonContent
<telerik:RadDateTimePicker TodayButtonVisibility="Visible" TodayButtonContent="Select Today/Now"/>
The final result is shown on the snapshot below: