In my frustration over the fact the Order of Ecclesia doesn't have a set year, I've concocted this insane fan theory. Take everything with a grain of salt. If anything, this is simply a series of coincidences that happen to line up favorably. I choose to believe it.
It's been firmly established that Reinhardt Schneider is a Belmont descendant. That's an indisputable fact. It's also been established that, as a child, Reinhardt was "often teased by his schoolmates, as they couldn't believe that he was actually directly connected to the famous bloodline."
This is where Serge comes in. Serge, like all the other villagers in Order, is a Belmont descendant as well. Perhaps the point in time where Serge/Reinhardt was being teased is slightly before Order, when there were villagers who weren't Belmont descendants, and couldn't believe that Serge/Reinhardt was a Belmont. It would make sense that he goes by a different name, so as to avoid teasing and disbelief.
Reinhardt was trained to use the Vampire Killer by his father, Michael Gelhart Schneider. Serge's father is nowhere to be found.
They're both similar physically: blue eyes, brown hair. Although Reinhardt's is lighter in color, this can simply be chalked up to age or a change in art style.
Reindardt was born in 1827. Serge is 10 at the time of Order. Thus, if this is to be believed, 1837 is the date. That works perfectly, as Legacy of Darkness takes place in 1844, and Circle of the Moon takes place in 1830. Castlevania 64 takes place in 1852, where Serge/Reinhardt is 25. All in all, 1837 really is the perfect year for Order.
I dunno. Everything lines up, albeit coincidentally, I'm sure. I like this theory, though, as it gives be a set year for Order, and that pleases my OCD.