change time of the marker key

Here is an example on how to manage all markers on the layer.

But you should be careful with setting new time. If you will try to shift you first marker in time later than the second marker, this code won't work correctly and to have a workaround here, you would probably need to create a multi-array with all markers, remove existing and set new shifted in time.

var comp = app.project.activeItem;  // current composition 

var myLayer = comp.selectedLayers[0]; // selected layer  
  replaceMarkers(myLayer, 1); // shift markers in one sec
function replaceMarkers(layer, shiftTime){  
var myMarkers = layer.marker;
if (myMarkers.numKeys == 0) {   alert("There is no markers on layer "+ layer.name); return false; }
    /*  LOOP THROUGH ALL MARKERS  */
    for (var i = 1; i <= myMarkers.numKeys; i += 1) {
        var m = myMarkers.keyValue(i);  // get first marker on the layer  
          
        var p = m.getParameters();  // object with marker's parametrs  
        var mTime = myMarkers.keyTime(i);// get time of the first marker  
          
        var newMarker = new MarkerValue(m.comment, m.chapter, m.url, m.frameTarget, m.cuePointName, p); // create new marker object  
          
        myMarkers.setValueAtTime(mTime+shiftTime, newMarker);   // set the same marker but one second later  
        myMarkers.removeKey(i); // remove the original marker 
    }
};