What time is it?

D. Indumathi, The Institute of Mathematical Sciences, Chennai

Can everybody's school start on the same day? What do you think will happen? Here is an imaginary conversation between a group of friends, all of whom live in different parts of the world but whose schools all happen to begin on the same day.
Hema was very excited. It was the first day of school after the summer vacations. In another hour she would be at school. She would exchange all the news with her friends. But first of all, she would have to go to a new building! Yes, she was going to middle school. She wasn't a baby any more. She shivered in anticipation.
There was an hour to go, and she was already ready. No-one telling her to finish her breaskfast or comb her hair, otherwise she would be late. How could one be late on the first day?
Hema stood on the balcony looking out. Chennai looked just the same as usual. There was nothing to see. She started pacing the floor. One hour was a long time. There was nothing to do. "Appa," she said. "Can I talk to Sunita on the phone? It's her first day too."
"Sure," said her father. "But keep it short; she's far away in Singapore." 
So Hema called Sunita. Her mother answered the phone. "But she's already gone to school," she said. 
Hema said, "But it's only just past 8:00." 
"No, Hema," said Sunita's mother. "It's past 10:30 here. We are to the east of you, you know. The Sun rose long ago for us!"
Hema put down the phone and told her father, "Appa, it's 10:30 in Singapore. Sunita has already gone to school."
"That's right," said her father. "You must have learned in school that the Sun rises in the East and sets in the West. So the Sun rises earlier in countries to the east of us."
"Then it must be even later in countries further east than Singapore," reasoned Hema. 
"You have a friend in Australia, don't you?" asked her father. "Why don't you call and find out?"
So Hema called Vijay, who lived in Sydney, Australia. She felt very important making international calls like this. Normally she wasn't even allowed to call her friends who lived in Chennai!
"Yes, hello, Uncle, I'm Hema speaking," said Hema into the phone. "Can you please tell me what time it is there?" Then, after a pause, "No, no, this is no prank, I'm really Hema from Chennai. I wanted to speak to Vijay but I knew he wouldn't be there."
She put down the phone, laughing, and told her father, "Appa, I was trying to save time by asking what I wanted to know, but Uncle couldn't believe that some one was calling to find out the time. And yes, they were just sitting down to lunch. He said it was a cold day and they were looking forward to some hot soup."
"I was also smart," continued Hema. "I asked him for the phone number of a friend in New Zealand: that's east of Sydney, you know."
So Hema called Sahir in New Zealand and reported to her father, "Sahir sounds really interesting: he's just back from school. It was a half-day because it was the first day of school. He couldn't believe that I haven't yet started for school. It's 2:30 in the afternoon for him."
The phone rang and Hema went to pick it up. It was Sahir.
"You won't believe what you will hear," he said. 
"What is it?" asked Hema. 
Sahir gave her a phone number and said, "Call Janet in Honolulu."
"That's the Hawaiian islands, right?" asked Hema. 
"Yes," said Sahir. "They are to the east of us, not far away, although in the northern hemisphere."
"So it must be late afternoon or early evening there, right?" asked Hema.
"Yes, it's about 4:30 in the evening, but that's not what amazed me. Call her yourself and find out," said Sahir. "Be sure to ask her if she went to school today."
Hema was mystified but called the number Sahir had given her. Janet seemed to have been expecting her call because she picked up the phone herself and started giggling into it.
"Is that Hema?" asked Janet. "Nice to make a friend." Then she giggled some more. "Will you promise to remember today as the day you made friends with me?"
"Of course," said Hema eagerly. "Today is also Monday, the first day of school. I won't forget. But what was it that made Sahir so surprised?"
"That's just it," laughed Janet. "I'm just going to go and play with my friends. It's the last day of holidays."
There was a silence while Hema digested this. "You mean 'today' is still Sunday for you?"
There were more giggles on the line. "That's exactly what I mean," said Janet. "We live on the other side of the International Date Line."
"I don't understand," said Hema. "What is this line?"
"Well, as the Earth goes once around, you get 24 hours of a day (day and night). But the Sun shines smoothly over different parts of the Earth. So you have to just draw a line from the North Pole to the South Pole and say that those to the right of this line are one day behind those to the left of this line."
"But how can you decide where to draw the line?"
"It was done long ago. They chose a line where there is the least amount of land to prevent confusion. So the Date Line is in the Pacific Ocean between Asia and America. But funny things happen in the region around the islands of Samoa, Tonga, Fiji and Kiribati. These are all to the south of Hawaii, between Australia and South America. They are all very close to the Date Line. But Samoa is to the right of the Date Line and the others are to the left. So for instance, Samoa and Tonga are exactly one day apart."
Hema was confused. "How can that be?" she asked.
"Well, the Date Line is just a convenience for someone very far away from it, like you are. But Tonga and Samoa are very close together and the Date Line goes right between them. So for instance, it is now about 3:30 in the afternoon in both Samoa and Tonga, but it is Monday in Tonga and Sunday in Samoa! And then it's Sunday in all places in America to the east of Samoa, including us in Hawaii."
"So if I go to Samoa I will get another day's holiday!" exclaimed Hema.
"That's right," said Janet. "That's true for all those in Africa, Europe, Asia and Australia, for whom it's mostly Monday."
"Yes indeed," said Hema's father, looking up from his newspaper. "But since you cannot go to Samoa, it's time for you to put the phone down and go to school. You've already learned a nice lesson, though, even before school began today!"
Notes:
1. The time zones are decided by a meridian called the Greenwich time meridian that passes through Greenwich in the U.K. Indian time is five and a half hours ahead of this.
2. This story is most easily read with reference to the world time map.
3. While "today" and "yesterday" can change in different places depending on the time of day (for instance, when it is Monday night in India, it will be Tuesday morning in Australia), the Date Line causes a DISCONTINUOUS time change between places just to its left and right, by an amount of 24 hours or one full day.