The calendar system can also completely ignore Minecraft days and use real-world time instead. Minecraft days will take 24 hours, the date and the season will match the real-life ones. It will be night when it is actually night and it will be daytime when it is actually daytime. Server owners can also disable the time sync and only sync the date and season (days will take 20 minutes again) if they prefer this. For details on how to do this and how you can change the time zone check out the configuration.
If you aren't using synchronize with real-time and your server has multiple overworlds with seasons enabled, using synchronize time of multiple worlds is recommended. The option can be found in the calendar.yml and it makes sure all worlds have exactly the same date, time and season. Perfect for servers with for example a lobby world.