Your question interested because as dumb as I feel, I didn't know that there was half hour time zones, so I researched it and found out why. The only way for me to explain it is to explain the purpose of time zones in the first place, and by the way I found out they are called "offset time zones" and they can be thirty minutes or even fifteen minutes. There is twenty four time zones in the world which are based on the fifteen degree increments of longitude. This is because the earth takes twenty four hours to rotate and there are 360 degrees of longitude so 360 divided by 24 equals 15. So the sun moves across 15 degrees of longitude in an hour. The offset time zones were designed to better coordinate noon as the point in the day when the sun is at its highest point in the sky, which is what our time zones revolve around... I hope this answers your question!
Reference
About.com: Geography - Offset Time Zones. http:/geography.about.com/od/culturalgeography/a/offsettimezones.htm.
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Half-hour time zones exist to account for regions that are not large enough to warrant a full one-hour difference but still have significant solar time variations. This helps maintain more accurate timekeeping in these areas relative to their solar noon.
Usually an hour, though in some places it can be half an hour.
There are 39 time zones because the Earth is divided into 24 longitudinal sections, each representing one hour of time difference. However, some regions choose to use a half-hour or quarter-hour offset from neighboring time zones, leading to 39 distinct time zones worldwide.
Generally there is one hour between time zones. However, in certain places this is reduced to half an hour
There are 24 time zones in the world, each representing a one-hour difference in time. However, some regions use half-hour or 45-minute offsets from this standard, resulting in more than 24 distinct time zones.
There are 24 time zones in the world, each one representing a one-hour difference from the next. This system helps organize the time across the globe based on the Earth's rotation. Some countries and regions may use half-hour or 45-minute differences to align their time zones with their specific geographical position.