Question 1

Which date format systems are supported?

Accepted Answer

Seven: strftime (C, Python, PHP, Ruby), Moment.js / Day.js, Luxon, date-fns v2+, Java SimpleDateFormat, .NET DateTime.ToString, and Unicode LDML / ICU. The tool formats a chosen date using any of these and cross-translates a pattern from one system to all others, so you can take a strftime pattern from a Python script and get the equivalent moment.js, date-fns, Java, .NET and LDML patterns side-by-side.

Question 2

Why does YYYY give the wrong year near December 31 / January 1?

Accepted Answer

In Java SimpleDateFormat, Unicode LDML and date-fns, capital YYYY is the ISO WEEK-NUMBERING year, not the calendar year. The week-numbering year can differ from the calendar year by 1 around year-end (because ISO week 1 is the week with the year's first Thursday). Use lowercase yyyy for the calendar year. This tool's footgun linter flags YYYY with a warning whenever it appears in those systems.

Question 3

What's the difference between DD and dd in date-fns?

Accepted Answer

It's the #1 date-fns footgun. Lowercase dd is day-of-month (01-31). Uppercase D is day-of-YEAR (1-366) — so DD gives a 2-digit day-of-year. If you write yyyy-DD-mm expecting 2026-15-01 you'll actually get 2026-015-01 (day-of-year). The linter flags DD and D in date-fns patterns to catch this.

Question 4

How do I escape literal text in each system?

Accepted Answer

strftime: only % needs escaping (use %%). Moment/Day.js: wrap literal text in square brackets, e.g. YYYY'[Q1]'. date-fns, Java SimpleDateFormat, and Unicode LDML: wrap in single quotes, e.g. 'yyyy-MM-dd'. Two single quotes in a row ('') produce a literal single quote. Luxon has no escape syntax — letters are always tokens. .NET uses backslash per-letter escapes, e.g. yyyy\-MM\-dd. The converter handles all of these automatically when translating between systems.

Question 5

What extra features does this tool have versus others?

Accepted Answer

(1) 7 format systems with 200+ total tokens. (2) Live preview against a chosen instant (UTC or local). (3) One-click cross-translate a pattern into all 6 other systems. (4) Footgun linter (YYYY week-year, DD vs dd, hh vs HH, mm vs MM, SS vs ss) with severity badges. (5) Token-by-token explanation of any pattern. (6) Token cheatsheet per system with live examples. (7) Best-effort format detection from a sample date string. (8) Code snippets for all 7 libraries (Python, moment, Day.js, Luxon, date-fns, Java, .NET). (9) Reference-date selector. (10) UTC toggle. (11) localStorage history (max 20, metadata only). (12) Shareable URL with pattern + system + UTC flag (fragment-encoded).

Question 6

Is my data sent anywhere?

Accepted Answer

No. All formatting, conversion, and linting run locally with JavaScript Date and Intl.DateTimeFormat. History is stored in localStorage on this device only and contains metadata (pattern + system + preview), never your date input.

Date Format Converter (strftime / token)

Preview

Token-by-token explanation

How to use

Preview

Token-by-token explanation