On the RadDateTimePicker with TabIndex set...only the CalendarPopupButton inherits TabIndex from parent picker while TimePopupButton does not.
So user will tab to the DateTimeTextbox and then tab to Calendar <a> tag...then tab order will skip over the TimeButton.
I have this workaround.
$(
function
() {
setTimeout(
function
() {
var
fromToDatePicker = $find(
"<%=RadDatePickerFromTo.ClientID%>"
);
var
fromToTimeControl = fromToDatePicker.get_timePopupButton();
fromToTimeControl.tabIndex = fromToDatePicker.get_textBox().tabIndex;