private void sortDrawablesOldest() {
        Sort.instance().sort(drawables, (DrawableData o1, DrawableData o2) -> {
            if (o1.file.lastModified() < o2.file.lastModified()) {
                return -1;
            } else if (o1.file.lastModified() > o2.file.lastModified()) {
                return 1;
            } else {
                return 0;
            }
        });
        refreshDrawableDisplay();
    }
Sorts by modified date with oldest first.
core/src/com/ray3k/skincomposer/dialog/DialogDrawables.java