Thai Date Format (พ.ศ. ↔ ค.ศ.)

Overview

Thai documents, forms, and casual writing use the Buddhist Era (พุทธศักราช, พ.ศ. = ค.ศ. + 543), not the Common Era Claude defaults to. A model without this skill routinely writes 2025 in a Thai government letter (should be ๒๕๖๘ or 2568), or converts a Thai year by guessing instead of the fixed +543 offset. This skill gives the exact offset, the three common display styles, and Thai numeral rendering.

When to use

• แปลงปี 2568 (พ.ศ.) เป็น ค.ศ., or the reverse
• เขียนวันที่แบบราชการ / business / casual ในเอกสารไทย
• Formatting a date for a Thai form, invoice, resume, or letter (often needed together with thai-invoice or thai-government-form)
• Rendering a number using Thai digit glyphs (๐-๙), e.g. for a formal document or engraving
• Parsing a Thai-formatted date string back into a usable date value

When NOT to use

• The task is pure English/ISO date handling with no Thai era or numeral involved
• The task needs Thai lunar/astrological calendar dates (Thai solar calendar with BE offset is a different system from the lunar calendar used for Buddhist holy days) — flag this distinction to the user rather than guessing a lunar date

Core knowledge

• Offset: BE = CE + 543. Example: 2025 CE = 2568 BE. This is a fixed arithmetic offset for the modern calendar.
• Historical caveat: Thailand moved New Year's Day from 1 April to 1 January starting BE 2484 (1 Jan 1941 CE). For any date on or after 1 Jan 1941, the fixed +543 offset holds for the whole year. For dates before that, Jan-Mar fell in the previous BE year under the old calendar — this skill does not model that historical edge case; flag it if a user asks about a pre-1941 date.

• Display styles:

  Style	Example	Where used
  Formal (ราชการ)	16 พฤษภาคม 2568	Government letters, official documents, formal invitations
  Business	16 พ.ค. 68	Invoices, receipts, business correspondence
  Casual	16/5/68	Chat, informal notes, UI date pickers
  ISO/CE	2025-05-16	Machine interchange, APIs, databases — never show this to a Thai reader as "the date" without conversion

• Thai numerals (๐๑๒๓๔๕๖๗๘๙): optional glyph set, mostly used in royal/ceremonial documents, some government forms, and stylized design. Default to Arabic numerals (0-9) even in Thai-language text — that is the actual norm in everyday Thai documents; only switch to Thai digits when the user asks or the context is clearly ceremonial/formal government.
• Month names: use the full form (มกราคม, กุมภาพันธ์, ...) for formal style, abbreviated form (ม.ค., ก.พ., ...) for business style — see convert.py:THAI_MONTHS_FULL / THAI_MONTHS_ABBR for the full list.

Common mistakes

1. Writing the CE year in a Thai formal document instead of converting to BE.
2. Getting the offset backwards (subtracting 543 from a CE year instead of adding).
3. Defaulting to Thai numeral glyphs everywhere — most real Thai documents use Arabic digits.
4. Truncating the BE year to 2 digits without checking the style calls for it (formal style always uses the full 4-digit BE year).
5. Confusing the Buddhist Era solar calendar (fixed +543 offset, same months as CE) with the Thai lunar calendar used for religious holidays — they are not interchangeable.
6. Applying the +543 offset to a pre-1941 date without accounting for the old April New Year.

Code

convert.py — no dependencies, pure stdlib:

• ce_to_be(ce_year) / be_to_ce(be_year) — year offset conversion
• to_thai_digits(value) / from_thai_digits(text) — Arabic ↔ Thai numeral glyphs
• ThaiDate.from_date(date).format(style="formal"|"business"|"casual"|"iso_ce", numerals="arabic"|"thai")
• format_date(date, style=..., numerals=...) — convenience wrapper

Run python3 convert.py for the self-test.

Known limitations

• Does not model the pre-1941 Thai New Year date shift (see Core knowledge above).
• Does not handle the Thai lunar calendar (religious holiday dates like Songkran-adjacent lunar observances need a separate lunar-calendar lookup, not this offset).
• Checked against the standard CE + 543 Buddhist Era rule, which is stable and not subject to legislative change — no re-verification cadence needed for the offset itself, but month-name spelling conventions should be spot-checked against a current Royal Institute Dictionary source if used in a legal document.