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Julian Date Converter
Convert between calendar dates and the Julian (ordinal) dates used in payments — YDDD as in the 23-digit acquirer reference number (ARN) and many DE 37 retrieval reference numbers, plus YYDDD and YYYYDDD.
Follows ISO 8601ordinal and week dates·verified Oct 2026· double-checked Oct 2026— every date 0001–9999 matches GNU date in five time zones; YYDDD uses the POSIX %y pivot; YDDD resolves to the latest such date not after tomorrow
YYYY-MM-DD ·
DDD = day of year 001–365 (366 in leap years)
Reading Julian dates
- Ordinal, not astronomical: in payments “Julian date” means the day of the year (ISO 8601 ordinal date), not the astronomical Julian Day Number.
- YDDD (1-digit year) — e.g. positions 8–11 of the 23-digit acquirer reference number, and the common YDDDhh… layout of DE 37. The year is taken as the latest year ending in that digit whose day DDD is not after today (one day of slack for senders in other time zones).
- YYDDD (2-digit year) — 69–99 → 1969–1999 and 00–68 → 2000–2068 (the POSIX
strptime %yrule). - YYYYDDD — unambiguous. Leap years: every 4th year except centuries not divisible by 400 (2000 is a leap year, 1900 and 2100 are not).
- The ISO 8583:1987 date fields DE 13, 15, 16 and 17 (and the date in DE 7, MMDDhhmmss) are MMDD, not Julian.
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