This pair is a good reminder that a zone comparison must be universal and date-aware rather than centered on a fixed city or memorized number.
A winter comparison
In winter, Europe/Belgrade normally uses UTC+1 while Europe/Moscow uses UTC+3. At the same instant, the Moscow clock is therefore two hours ahead. If it is 09:00 on January 15 in Belgrade, the corresponding local time in Moscow is 11:00 on the same date. The result follows the zone rules for that exact winter instant.
A summer comparison
In summer, Belgrade normally moves to UTC+2 while Moscow remains at UTC+3. The difference becomes one hour. If it is 09:00 on July 15 in Belgrade, Moscow reads 10:00. Nothing about the cities’ positions changed; only one zone’s seasonal offset changed. This is why a single permanent answer for the pair would mislead.
The reusable lesson
Select named zones, keep the intended date, and calculate from one instant. Do not treat Moscow, Belgrade, or any other location as the default truth for every user. The browser zone can fill the first field as a convenience, but the user must be free to replace both zones. The server then supplies the clock and zone database used for the answer.