latitude and longitude
There are 15 degrees of longitude in each standard time zone
There is no such longitude. The maximum degrees for lines of longitude is 180 east and west only.
360° of longitude divided by 24 time zones gives 15° per time zone (average)
No line of longitude bends, but the time-zone boundaries are defined to depart from the meridians in order to accomplish that.
It is a time zone.
earths rotation
The time zone centered at 120° west longitude is UTC-8.
That would be zero degrees longitude.
There are 24 time zones. Divide 360 degrees longitude by 24 and you get 15 degrees for each one-hour time zone.
earths rotation
There are 24 time zones. The Earth is 360 degrees around the circumference. So, for every 15 degrees of longitude there is one time zone.