What year will it be in the 31st centry, because isn't that like 300 years from now? How did that make any sense how could everybody still be alive and that being Rini's time period?

Yeah, it's gonna be 3113 year when Rini's born. I've got another question here then: how could Mamoru and Usagi not to make love for so long period? ^-^

That doesn't make sense to me either. The anime explained that a disaster had struck the earth, putting it through a second Ice Age. Until finally in the 30th century, Neo Queen Serenity came to the throne and used the powers of the Silver Crystal to reawaken the planet. Where was she all that time? @_@

In the manga, Usagi became queen at age 22 and since then on, because of the power of the Silver Crystal, she and the rest of the Sailor Team stayed their age for 10 centuries. Chibiusa is said to be over 900 years old in the manga but for some reason, she's still a little kid. @_@

Multiple times in the show the senshi are reincarnated. That obviously keeps happening throughout the centuries. Chibi-Usa is their future child when they are reincarnated as Neo-Queen Serenity and King Endymion.

It explains that Serenity used the silver crystal to extend the lives of every one on earth, and you only show age every 100 years or so. So if Rini is about 900, then she only show to be about 9.