private static long calculateTimeStamp(final VirtualFile[] files, Project myProject) {
    long timestamp = 0;

    for(VirtualFile file:files) {
      if (file == null || !file.isValid()) break;
      final PsiFile psifile = PsiManager.getInstance(myProject).findFile(file);

      if (psifile != null && psifile.isValid()) {
        timestamp += psifile.getViewProvider().getModificationStamp();
      } else {
        break;
      }
    }
    return timestamp;
  }
private static long calculateTimeStamp(final VirtualFile[] files, Project myProject)