There was a time when the 'todo list' of what happens during a tick was static. That is, for EVERY tick, ALL objects, repops, area echoes, NPCs, timers, scripts etc. were ALL updated.
Nowadays there is a lot more intelligence involved, so what happens on any given tick could vary from 0.1 seconds worth of processing right up to a second or so if there is something really big going on.
But even my knowledge is second-hand, because the connectivity between the server and the player or immortal is such a major contributor to the 'tick lag' that you would 'notice'... so often people complaining about the lack of CPU grunt in the server is actually someone else in their street doing some heavy downloading, etc.
Only Dulrik would be able to quote actual processing times from the server, should he ever care to. Until then, feel free to donate so we can upgrade to a dedicated server.
The time-of-day in-game is not something that gets updated every single tick, because it only changes at certain IRL times and is hence not related to when or how long the ticks code runs (refer to
http://www.shatteredkingdoms.org/time to observe this IRL-to-IC time calculation in practice).