@Condandy, if you want to manage everything in RW, just go with Gregorian. You can set the year to whatever you want. If you don't, you have to manage calendars outside of RW. I have a number of customer calendars for my realm, which I track in a spreadsheet. In the beginning, I would copy dates over, but now I just keep them in a separate spreadsheet and made the Gregorian calendar the one used by one of the realms in my campaign. I enter everything in Gregorian.
Having custom calendars ads some nice flavor but rarely is crucial to a game. Most players can't be bothered to track date at all, much less on a fantasy calendar. It is almost always a DM task. It's nice to give some flavor, but you have to balance that with the extra work it places on you as the game master.
What you certainly do not want to do is start a campaign dependent on custom calendars and put it into Realmworks hoping that it will be supported soon. Lots of us have been waiting years.
While I would love to have custom calendars so that I had an easy way to make my games more immersive with little work, I have come to terms with the fact that this is not going to be possible in the near future. Instead, I enter all dates in Gregorian, so that I can take advantage of tools like the timeline feature in Realmworks, knowing that if they ever do release custom calendars, I'll have those dates entered and can convert them with the tool. And if they don't every release custom calendars, it's a bummer, but it doesn't prevent me from taking advantage of all the other features of the tool.