mathematics - The punctual worker and the stingy boss

Tom works with a stingy boss that will deduct from his salary for every minute that he's late for work. Unfortunately, Tom has to pass by the boss's office on the way to his cubicle. Being a lazy worker, Tom decides to arrive at the office exactly 1 minute before his boss every day. Tom does not know when his boss will arrive at the office as the boss's office is located far away from Tom's cubicle. One thing that Tom knows is that the boss will arrive at the office at the same time every day, no sooner, no later. Tom can pull off the "go to the washroom" trick once every day to pass by the boss's office twice (to the washroom and back from the washroom with a 5-minute interval). How many days will it take for Tom to work out his boss's time of arrival?