One hour wasted or saved

A visitor came to a research lab and enquired about their guide to a group of students. A junior student said "Could you come after one hour, you will meet Sir". The visitor went away. A slightly senior student told the junior student "Why did you tell the visitor to come after one hour? Do you know for sure Sir will come in one hour? Otherwise the visitor will waste one hour. To this a senior student replied do not worry "If the visitor does not meet Sir when he comes after one hour he would have wasted one hour but if he met Sir he would waste two hours for sure!"