Calendar button on the right

[fabdurso]

Everything is working fine with datetimepicker (using it with Simple form).
But without having changed anything on the default CSS files, the calendar button is on the right of the page as shown here:

What am I doing wrong?

[fabdurso]

I quite fixed it by adding

.input-group-btn {
  width: 100%;
}

to bootstrap-datetimepicker.css but I'll wait from an official answer from @zpaulovics in order to close the issue. I'd really appreciate it and it could help more users IMO.