During the second half of the year, California is 8 hours behind Britain, and Irkutsk (Russia) is 8 hours
ahead of Britain. So Irkutsk should be 16 hours ahead of California. Why is Irkutsk listed as being only 15 hours
ahead of California during a single day in October?
California and Irkutsk both put their clocks back on the last Sunday morning in October.
But Sunday comes 16 hours earlier in Irkutsk, so for 16 hours Irkutsk has put its clocks back and California hasn't.
This 16 hour period is Sunday in Irkutsk, but Saturday in California.
So relative to Irkutsk, California is only 15 hours behind for a single Sunday in October.
Relative to California, Irkutsk is only 15 hours ahead for a single Saturday in October.
In compiling this list, I have made the arbitrary decision that
- places whose winter times are less than 13 hours apart are on the same day (e.g. UK and
Irkutsk, or UK and California)
- places whose winter times are 13 hours apart or more are on different days (e.g. California
This causes many odd effects, for instance when comparing the time in various Russian places with respect to
Greenland, but it is the best way of meeting the needs of people who phone companies around the world in business
hours. Another solution would be to state the exact time when the clocks change in the other country, but that
would vastly increase the amount and complexity of data, because it would then become necessary to clutter the
list with unimportant details such as the fact that Moscow is three hours ahead of London all year around except
for two hours in the middle of the night on two days of the year.