code: function() {
  
          return Q.all(this.tech
              .getPaths(PATH.resolve(this.root, this.output), this.tech.getBuildSuffixes())
              .map(function(path) {
  
                  return QFS.lastModified(path)
                      .fail(function() {
                          return -1;
                      });
  
              }))
              .spread(Math.min);
  
      }
docs: Get files minimum mtime in milliseconds or -1 in case of any file doesn't exist.
  
  @return {Promise * Number}