Selapan

The problem

Across Java, families use a person's weton, their birth-day in the Javanese cycles, to plan naming ceremonies, weddings and the days of remembrance after a death. It looks like a table lookup. In fact it combines four cycles: the 5-day pasaran, the 7-day week, the 210-day wuku, and a lunar year that begins in 1633 CE. A correction system called the kurup also shifts the lunar count every 120 years.

Existing weton sites are ad-supported, cite nothing and disagree with each other. They fix the current kurup, so historical dates come out wrong. They convert dates before 1633, when the lunar system didn't exist. They show no working, so a reader can't catch a mistake. Communities that follow the older Aboge rule sometimes observe Idul Fitri a day after the national date. No site shows that difference.

What I built

Every answer shows its working

Each figure on the page opens to show how it was computed: the reference date, the number of days since, the cycle length, the result and the source. Anyone can redo the answer by hand from the screen. I built the calendar engine so it returns this working for every value. A figure without it can't be displayed at all, so the page can't drift from the arithmetic.

Reference dates come from cited sources

Each cycle is counted from a reference date that lives in a data file with its source and where to find it. The build fails if a reference date has no citation or doesn't match its own stated date. Where a value is inferred from other sources, it is marked unverified, shown in grey, and listed on a public sources-and-gaps page.

It refuses dates it can't answer

Each cycle has its own valid range. Ask for a date in 1500 CE and the lunar calendar declines, with the reason, while the pasaran and wuku still answer, because those cycles run continuously. I counted every date as a plain whole number of days, with no time zones or clocks involved. The same input always gives the same answer.

Aboge and Asapon side by side

The divergence view reckons the same date under both rules and shows the mechanism that separates them. For one example date, Asapon reads 21 Sapar and Aboge reads 20 Sapar. Aboge communities see their own reckoning next to the national one, and the site doesn't call either one wrong.

Calculation and interpretation are kept apart

Weton feeds primbon: character readings, marriage matches and auspicious days. Many people take it seriously and many reject it. I gave computed values one colour, indigo, and traditional material another, a red taken from printed almanacs. I held back the interpretation layer entirely. Its rules already run at build time, but it won't ship until a reviewer familiar with Javanese practice agrees to check it.

Result

Selapan is live in Indonesian and English, with seven pages in each language. Beyond conversion, it has cycle wheels that realign every 35 days (the selapan) and every 210, a wall-calendar month view and the traditional farming calendar (pranata mangsa). It also counts the dates of the mourning ceremonies as plain cited arithmetic, with no interpretation. It checks exact cycles over 36,524 consecutive days. The site presents itself as a personal project, not an authority, and says that practice varies by region. I built it in one day, on 6 August 2026.