ICU-11533 Myanmar / Burmese calendar support

[mapmeld]

Background

A while ago I proposed PR #630 to add the Myanmar lunar-based calendar (aka Burmese calendar) to ICU. This calendar is frequently used for birthdays and on official documents.
After difficulties with supporting and testing a complete calendar, I closed the PR in 2024.
(Last year I suggested changing only the year in #3482 , which was rejected)

The reference for this work is https://github.com/yan9a/mmcal , an open source C++/JS project which gave their permission.

What's new?

• I squashed my previous work and tests into one commit.
• I gave Copilot access to the mmcal JS file, my previous work, and my existing tests. This addressed two problems which I have had for a long time: defining the min and max calendar limits, and implementation of leap years.
• Final commit is cleanup after rebasing to 2026 and removing comments/whitespace (also AI assisted)

On AI assistance:

• The original tests and framework are still there. The JS spec is a human-written reference.
• From the beginning of this process, I have said that my obstacle was understanding the test system, both for parsing 1000s of lines of limit failures, secondly for testing this PR and not the full test suite. This made my dev process / iteration very slow.
• For developers who aren't familiar with this process, my process includes reverting and pushing back on AI recommendations until the approach is reasonable and minimal.
• Finally, I have written this message you're reading now without consulting the AI. I've reviewed the files changed by the PR. If you see the previous PRs it is very similar. I will continue communicating with you personally.

Checklist

• Required: The PR title must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Required: Each commit message must be prefixed with a JIRA Issue number. Example: "ICU-NNNNN Fix xyz"
• Issue accepted (done by Technical Committee after discussion)
• Tests included, if applicable
• API docs and/or User Guide docs changed or added, if applicable
• Approver: Feel free to merge on my behalf

[srl295]

this should go into CLDR-8081 first, as this file (root.txt) is generated from there.

[mapmeld]

I faintly remembered I had a PR for CLDR. I am updating the commit to add names for the months to my.xml
unicode-org/cldr#3806

[srl295]

you mentioned issues with testing. Note that ICU4C testing can use json now, https://github.com/unicode-org/icu/blob/main/icu4c/source/test/intltest/messageformat2test_read_json.cpp for example. So the test data could be in a json file if that helps.

[srl295]

oh and hi again 👋

[richgillam]

Is there a ticket filed for this change, and can you link it to this PR?

For CLDR it's CLDR-8081 and the PR is https://github.com/unicode-org/cldr/pull/3806/changes

The CLDR Design Working Group would like to see a formal proposal for this new addition, and for the structure of the necessary data in in CLDR and don't want to see this merged in the meantime.

The PR has a description.

[DraganBesevic]

I have a couple of questions. What is the reference source for this calendar you used to verify you're getting correct dates? Did you verify it over the wider range of dates, or only spot checked?

Also, I would like to see some unit tests for add() and roll(). I noticed that you didn't add any overrides for those two methods, so I am curios if that works as is, especially at leap years' boundaries and similar