Open a demo page https://demos.telerik.com/aspnet-ajax/datetimepicker/overview/defaultcs.aspx?skin=Bootstrap and select Bootstrap skin. You will see that the right corners of the clock button are cutted down.

Workaround:
.RadPicker .rcSelect {
    border-top-right-radius: 4px !important;
    border-bottom-right-radius: 4px !important;
}