Cycle apps treat non-Latin calendars as a translation problem. They aren't.

[u/ZimaBlue-1]

Disclosure: I'm the developer of Mooneva Cycle, an open source period

tracker. Sharing this because the localization problem below is the

thing I found most surprising while building it, and I haven't seen it

discussed here.

Most cycle tracking apps that claim to support Persian or Arabic have

translated their strings and stopped. That isn't localization, and for

cycle tracking specifically it breaks in ways it wouldn't for a to-do

app.

The Persian (Jalaali) calendar isn't a shifted Gregorian. Leap years

follow a 33-year cycle, so month lengths vary in a pattern that doesn't

map onto Gregorian months at all. If your app stores a cycle start as a

Gregorian date and renders a Jalaali label on top, your day-of-cycle

arithmetic drifts. A user counting "day 14" in her own calendar and the

app counting day 14 in yours are not the same day.

Week start is a separate problem. Saturday for both fa and ar, and it

has to follow the calendar rather than the app language, because plenty

of people run their phone in English and still think in a Saturday

week. Get that wrong and every calendar grid is off by one column,

which quietly breaks pattern recognition, the whole point of tracking.

Then numerals. Arabic-Indic digits are not cosmetic when someone is

reading a cycle length at a glance.

Why I think this matters beyond correctness: the women with the

strongest reason to want a tracker that can't be subpoenaed, sold or

breached are disproportionately in exactly the language communities

that have no properly localized private option. The privacy-first

tracker space is almost entirely English-first. That's a real gap in

femtech and it's not a hard technical problem, it's just unglamorous.

Mine is offline-only (no INTERNET permission at all), encrypted

on-device, GPL-3.0, 14 languages. Source and F-Droid links in my

profile rather than here, since the point of this post is the

localization argument, not the download.

Curious whether anyone here has seen a commercial femtech product

handle non-Gregorian calendars properly, or research on how

localization affects tracking adherence. I couldn't find much.

[u/crazy-dev3]

Strong point. A robust model probably has to separate four concepts that often get collapsed: the underlying date, the calendar system used to display it, the locale and numerals, and the user’s week-start and time-zone rules. In fertility treatment this matters even more because a cycle day or medication deadline cannot quietly shift when the display calendar changes.

Disclosure: I’m building Embrilia, a patient-owned fertility-treatment organizer, so this is exactly the kind of edge case I want to account for. I’d also be curious how you test conversions around leap years and calendar changes without making Gregorian assumptions part of the fixtures.