This Mod always shows the Christian / Gregorian Date next to the Normal Game Date and resized the box for the dates accordingly.
Should be Ironman / Achievement Compatible, well should.
UPDATE Ironman / Achievement Compatibility, Quote Johan:
Sadly with the powerful moddable UI system UI files and localisation all those files are checksummed.
Basically it seems that I:R cannot be modded in a way that allows Achievements.
Yes, it says BCE. This was not a decision on my part, the mod just shows the date that is usually found in the tooltip in the date field. I am not sure how to change BCE to BC nor if it is possible. So please stop ranting about it in the comments.
If you really can't live with BCE, play in German, the german version uses the german translation of BC (;
Also some people were insulting me for taking credit for work from the Devs. To make it clear, this is not a big effort on my part and I never claimed it was, I explicitly described the minority of the changes when asked about Achievement compatibilty, it just resizes the date box and copies the gregorian date from the tooltip that is in the base game into the resized box. It's a small mod, but something I felt a lot of people might like to have. If that annoys you, be annoyed.