I was born in JAPAN at 10 pm GMT. Travelled to NewYork on my 15th birthday by an aeroplane. Now, I am 20 years old today. What's my exact age?
There is two timezone in London BST(British SumMer Time) which advances one hour in summer, which is from April To October and work as GMT + 1 hr. Other one is GMT , which is in winter. So in September you are 8 hours behind but actually you are 9 hours behind.
You are 20 years and 9 hours old. As you are in London, UK which is 8 hours behind of japan, which means you think that at 10 pm in London, you become 20 years but you are actually 20 years and 9 hours for exact.
JAPAN is 8 hours ahead of UK. Thus, at 2 PM in UK you will have 20 years. But before 2 PM you are still 19 years old. https://www.timeanddate.com/worldclock/converted.html?p1=248&p2=136
If you are in London (as mentioned in the comment) and claim to be 20 years old today, then your actual age would be:
Either 20 years old or 20 years and one day.
Let's assume you were actually born on 19th September 1997 at 10pm GMT. This would be the early hours of 20th September 1997 local time. You would regard your birthday as 20th September.
In traditional Japanese culture people used to count their age starting from age one in the year they were born (kazoedoshi). This practise has largely been abandoned in favour of counting ages western-style, based on the number of years that have passed since birth. You are unlikely to still be following this tradition as you would also be celebrating your change of age on 1st January, not in September.
You flew to New York on your 15th birthday, crossing the international date line. You would land on the day before your birthday. This is irrelevant as you would still regard your birthday as 20th September.
Now you are in London, and are celebrating your birthday on 20th September. Depending on the time of day, it will either be 20th September or 21st September in Japan. So you are either 20 years old or 20 years and one day.