Adding a month is a footgun even without time zones. I was checking a spreadsheet against another implementation month by month and found that HyperFormula 3.4 returns Feb 28 for EDATE(Jan 31 2028, 1). Excel, Google Sheets and Numbers all return Feb 29, because 2028 is a leap year. Jan 31 + 1 month in a leap year is now a test case in anything I write that does month math.

Thank you for pointing this out with HyperFormula! Appreciate you testing in various environments and sharing the results. I've confirmed the issue and submitted a PR with a fix that should be in the next release.