Google Web Designer (workaround): insert time in front of labels and events at some point on the timeline.

rejig_timeline_labels_events.js

var AT_SECONDS = 0;
var INSERT_SECONDS = 0;

// Replace the string below with the text of the element containing labels and events that need to be shifted.
var STR = `

`

// Grab string of timeline labels from attribute 'data-gwd-animation-labels' for an element.  Make 
// timeline space 'at_seconds'.  Insert 'insert_seconds' of space.  Console output are the new labels.
var rejig_labels = function (what, at_seconds, insert_seconds) {
    var at_millis = Math.floor((at_seconds * 1000) / 20) * 20;
    var insert_millis = Math.floor((insert_seconds * 1000) / 20) * 20;
    var whats = what.split(';');
    var label_millis_map = {};
    var millis_label_map = {};
    var output = "";
    whats.forEach((val, i) => {
        var key_val = val.split(',');
        var millis = key_val[1];
        if (millis >= at_millis) {
            millis = parseInt(millis) + parseInt(insert_millis);
        }
        label_millis_map[key_val[0]] = millis;                
    })
    Object.keys(label_millis_map).forEach((k,i) => {
        millis_label_map[label_millis_map[k]] = k;
    })
    Object.keys(millis_label_map).sort((a,b) => parseInt(a) - parseInt(b)).forEach((k,i) => {
        output = output + (output.length > 0 ? ';' : '') + millis_label_map[k] + ',' + k;
    })
    return output;
}

var rejig_events = function (buffer, at_seconds, insert_seconds) {
    var at_millis = Math.floor((at_seconds * 1000) / 20) * 20;
    var insert_millis = Math.floor((insert_seconds * 1000) / 20) * 20;
    var matches = {}
    var eventRegexp = /data-event-name="(.*?)"\s*data-event-time="(.*?)"/g;
    var match = eventRegexp.exec(buffer);
    while (match != null) {
        var value = match[2];
        if (value >= at_millis) {
            value = parseInt(value) + parseInt(insert_millis);
        }
        matches[match[1]] = value;
        match = eventRegexp.exec(buffer);
    }

    for (var match in matches) {
        var replacementRegexp = new RegExp('data-event-name="'+match+'"\\s*data-event-time=".*?"','g');
        buffer = buffer.replace(replacementRegexp,'data-event-name="'+match+'" data-event-time="' + matches[match] + '"');
    }
    return buffer;
}

var rejig = function(str, at_seconds, insert_seconds) {
    // rejig labels
    var labelsRegexp = /data-gwd-animation-labels="(.*?)"/g;
    var match = labelsRegexp.exec(str);
    if (match != null) {        
        var replacement = rejig_labels(match[1], at_seconds, insert_seconds);
        var replacementRegexp = new RegExp('data-gwd-animation-labels="(.*?)"','g');
        str = str.replace(replacementRegexp, 'data-gwd-animation-labels="' + replacement + '"');
    }

    // rejig events
    str = rejig_events(str, at_seconds, insert_seconds)

    return str;
}

console.log(rejig(STR, AT_SECONDS, INSERT_SECONDS));

Work with the "Code View" in Google Web Designer: into STR (above) copy the HTML for the element with the target animation timeline where you need to shift labels and events by some number of seconds. This is the element with the data-gwd-animation-labels attribute and the event divs with data-event-time attributes. Replace AT_SECONDS and INSERT_SECONDS with desired values in seconds.