I can imagine two different approaches:
- You create a calendar consisting of eras that represent your desired system.
- You use a preset calendar and add one or more additional properties to your event type, which contain the date of one or more additional calendars.
We demonstrated an example of approach 1 last year with a zodiac calendar. In this case, I used a script to create a project template in AT2 format, which could then be imported and converted with AT3. If the time periods in your project are manageable, you can of course also enter the eras manually in your calendar definition. You would then need to obtain the data for this from an online Panchanga calculator.
For approach 2, I also have an example that I use regularly myself: My events have a moon phase property. Since I still use AT2, whose file format is easily accessible to me, I can generate the moon phases from a Python script and have them written to the project file. With AT3, however, something like this could work via csv export and reimport.
However, there are other solutions as well. It may be helpful to have a look at the corresponding thread in this forum.