Moving either the moveStart will effect move end. End position may not be behind the start position. If ,oveStart is move to a position within a textarea moveEnds value will also move to that place if it has not been set e.g.
var range = document.getElementById("aTextArea").createTextRange();
range.collapse(true);
range.moveStart('character', offsetIntoTextarea);
range.moveEnd'character', offsetPassedStart);
range.select();