While time zones are based on lines of longitude, some lines are drawn to avoid populated areas or to follow borders, and some countries have added their own variations because they don't want to be divided into several zones. In a few regions, the time kept is not one of the 24 standard time zones because half-hour or quarter-hour differences are in effect there. Variations also occur because different countries or regions have adopted daylight saving time.
China is the largest country with only one time zone (it should span five). That would be like New York, Chicago, Denver, and Los Angeles all being in the same time zone. India is the second largest country with only one.
Russia adheres to its standard time zones except the entire country is on permanent daylight saving time and so is an hour ahead of their actual zones.
Australia uses three time zones; the eastern and western zones adhere to their assigned time, but the central zone is a half-hour ahead. In the Middle East and South Asia, several countries also use half-hour time zones.
In South Asia, if you follow a straight line west along the 27º latitude you will move back and forth across time zones: from Pakistan UTC +5 hours, India +5:30, Nepal +5:45, India (Sikkim) +5:30, China +8, Bhutan +6, India (Arunachal Pradesh) +5:30, Myanmar +6:30.
Russia has the most time zones (11), followed by the U.S. with nine (six for states and three for territories), and Canada with six.
You can actually be in three time zones at the same time, at the spot where the borders of Norway/Finland, Norway/Russia, and Russia/Finland meet.