The easiest thing for me was to have a separate spreadsheet where I have a my calendar conversions. In RealmWorks, I just entered the first date of the campaign as the in-real-life date of the first session, in Gregorian. Then I just backdated or added new dates from there. The reason I started to do this is so that if/when custom calendars are ever revealed, I will already have many dates entered and can then toggle to the custom calendar I need.
This works fine for contemporary in-campaign events, but if I want to enter dates far into the past or future, I usually don't bother converting them as I've been too lazy to figure out how to properly do the calendrical calculations. I've been tempted to go on Guru.com and hire someone to make a simple app that will allow me to enter a date in the format of one of my custom calendars and have it provide the date conversions for all my campaign calendars.
But, I'm saving up for a 3D printer...and it would be nice to have a display to lay on the table as a digital battlemap, and there are always minis to buy, and I have this bad habit of collecting cartography software, and there are battelmaps to buy, and cool kickstarters to back...
With so many things I would like to spend my hobby money on, making my own custom calendar-conversion app is a low priority.
Had I had more foresight, I should have just selected a few historical and little known contemporary calendar systems and just renamed them for my campaign (e.g. the Bahai calendar, Mayan calendar, Chinese lunar, Kurdish, etc.) But I'm already too deep into using the calendars I've selected to redo everything.
