Tauren and trolls both count in base 12. Since ancient times, counting on their hands was practiced by pointing the thumb at three points on each of the other two fingers: the base, joint, and tip, for six points on each hand.
The Zandalari pioneered the 24-hour clock by dividing both daytime and nighttime into 12 hours each. The Zandalari were also pioneers in ancient development of mathematics (especially geometry) and astronomy, which, besides the ubiquity of triangles and circles in Zandalari architecture, influenced the division of the circle into 360 degrees, made of twelve 30-degrees sectors each symbolizing a 30-day month. (The ancient Zandalari calendar had exactly 360 days in a year; by the time they realized their error, multiples of 12 had already been firmly entrenched in their culture.)
When the dark trolls of central Kalimdor became the first night elves, they retained 24-hour timekeeping despite eventually adopting the decimal system for counting. The retention of multiples of 12 in night elf timekeeping was helped by the division of the year into approximately 12 lunar cycles. The night elves historically used a lunisolar calendar with solar years and lunar months, with the Lunar Festival being the start of the year. From the Kaldorei Empire, this calendar was passed to Pandaria, replacing its previous calendar that was too heavily entrenched in mogu tradition. The quel’dorei of Quel’Thalas, however, switched to a fully solar calendar when they cut their ties with Elunism.
Titanic races (vrykul, earthen, and tol’vir) had used decimal counting from the start, having five-fingered hands. The exception were mechagnomes, who used the binary system and systems derived from it, such as hexadecimal.
The humans of Arathor inherited a lunisolar calendar from their vrykul ancestors, but eventually switched to the modern solar calendar under cultural pressure from Quel’Thalas, which at the time was the dominant source of scholarly knowledge for the fledging human civilization until the establishment of Dalaran. The solar calendar of Quel’Thalas is effectively the one in use today throughout Azeroth, thanks to human expansionism. It is different from the ancient Zandalari calendar by its variable number of days in a month (31, 28/29, 31, 30…), whereas the Zandalari calendar used 30-day months, followed by five or six transitional days at the end of each year that were not part of any month.