A small update
This solution does not work when RadDatePicker is bound in edittemplates in grid and such. Also, if you have multiple RadDatePickers (like date from and date to) you might end up with a huge switch/case or if/else if/else which is not really scalable.
My solution was to change the onClick function so it takes in a parameter.
This is somewhat of a "hack" but since there is only one popup calendar available when you open up your the datepicker, just select the first one. That leaves you with the RadCalender and you can set the date from there.
Btw - it's 2017. Telerik should add this function to calendar pick both server and client side.
_calenders = $(
todayTriplet = [_today.getFullYear(), _today.getMonth() + 1, _today.getDate()];