Date Format Pattern Cheatsheet (dayjs / strftime)

Date-format queries reveal more about your work than you might notice

Searching for %Y/%m/%d %H:%M:%S tells an observer not only that you are writing code, but which language stack (Python / Ruby / PHP), what output shape you need, and roughly which part of a system you are building. Online date-format tools that send queries to a server accumulate a log of who looked up which patterns, how often, and when. Individual queries look mundane; repeated patterns across sessions start to sketch a picture of your project rhythm and your team’s technology choices.

Ad-supported developer tools routinely pass behavioural signals to targeting platforms. A classification like “backend engineer frequently looking up ISO 8601 timestamp patterns” becomes an audience attribute, which travels with you across the ad ecosystem well beyond the tab you closed.

Keeping your development activity out of external logs

Pulling documentation while coding is entirely normal — the problem is when the frequency and content of those lookups are continuously transmitted to a server you do not control. A query like “RFC 3339 YYYY-MM-DDTHH:mm:ssZ format output” contains implicit context about which external specification you are trying to satisfy, and therefore which integration you are working on. Routing reference lookups through a browser-only tool is a small habit that keeps your development log local.

How the Date API and string replacement keep everything local

This tool takes the current time from the browser’s Date object (or a custom reference datetime you provide), then resolves both dayjs / Moment.js-style tokens and strftime-style tokens through JavaScript string replacement — no external API call is involved. The timezone comes directly from Date.prototype.getTimezoneOffset(), so your local UTC offset is reflected without the tool ever learning your location.

Open the DevTools Network tab and type a pattern: after the initial page load, no further requests appear. The implementation is on GitHub for anyone who wants to verify the replacement logic.

Using the presets as a reference checklist

The ten built-in presets (ISO 8601, RFC 3339, RFC 2822, Japanese date, US date, log line, and others) exist to answer the recurring question: “does my output match the spec I’m targeting?” Before writing a formatter from scratch, drop the spec’s pattern into the tool and check that the rendered output looks like what the downstream API or log system expects. Catching a misplaced mm (minutes) vs MM (months) during development is much cheaper than finding the mismatch in production.

Why dayjs and strftime use different token grammars

dayjs / Moment.js tokens use repeated letters to express width: YYYY for four-digit year, MM for zero-padded month, M for the bare integer. strftime, which dates back to ISO C’s <time.h> and survives in Python datetime.strftime, PHP date(), and Ruby Time#strftime, uses percent-prefixed single letters: %Y %m %d. This tool implements the two grammars as independent string-replacement passes, switched by the Mode toggle, so a stray %Y inside a dayjs pattern stays literal rather than being interpreted.

The most expensive mistake in both grammars is letter case. In dayjs, MM is month and mm is minute; DD is day-of-month and dd is the day-name abbreviation. In strftime, %M is minute and %m is month — the inverse of the dayjs convention. %Y is a four-digit year and %y is two-digit, which catches people writing log timestamps that suddenly look ten years old. The presets sidestep this entire class of bug by carrying the spec-correct casing.

Timezones, UTC offsets, and locale-dependent tokens

ISO 8601 and RFC 3339 both accept YYYY-MM-DDTHH:mm:ssZ, but RFC 3339 is the stricter profile and is what most APIs actually require. The browser’s Date.prototype.getTimezoneOffset() returns the local offset, so producing a literal Z (UTC) output usually means either feeding the tool a UTC reference moment or post-processing the +09:00 offset back to Z.

Locale-aware tokens add another wrinkle. MMMM (full month name) and dddd (full day name) return January / Monday in English, 1月 / 月曜日 in Japanese — driven by the browser’s Intl.DateTimeFormat and navigator.language. For log files aggregated across regions, sticking to locale-independent numeric formats (YYYY-MM-DD HH:mm:ss) avoids downstream confusion when a parser written in one locale reads timestamps written in another. To round-trip the same instant between epoch seconds, ISO 8601, and Date, unix-timestamp shows all three values in lockstep, and iso-week makes the YYYY-Www boundaries explicit when working with week-numbered formats.