I am not denying that there are far more than 24 time zones, just that the odd balls that are not +/- a whole number of hours, or where that value is more than 12 are not major ones. For example NATO uses letters A-Z (except J) to designate time zones. D is +4 E is +5 and F is +6, but there is no letter for the ones any of the others there is the convention to follow the letter with a "*" to designate half an hour pase i.e. D* is +4:30 and E* is +5:30 and if you really want to get into the weeds a dagger and a double dagger are used for the :15 and :45 versions. Either there is no division between major and minor time zones in which case the term phrase is useless. I guess someone could decide that the definition was base on the number of people who live in them, in which case you could aruge it down to two +5:30 and +8 as they are the only time zones with more than 1 billion residents or up to as many as you like (assuming that Saudi Arabia hasn't officially adopted something other than local solar time as the standard). I guess you could stretch major to include the couple of places that are more than 12 hours ahead of GMT/UT/Z but then you would have something like 27. In any case 24, without making a strange and arbitrary decision about what consititutes a major time zone, is not a good number.