src/Pure/General/time.scala

   def max(t: Time): Time = if (this > t) this else t
 
   def is_zero: Boolean = ms == 0
   def is_relevant: Boolean = ms >= 1
 
-  override def toString = Time.print_seconds(seconds)
+  override def toString: String = Time.print_seconds(seconds)
 
   def message: String = toString + "s"
 }