Is strftime portable across languages?

Mostly yes for POSIX tokens like %Y, %m, %d, %H, %M, %S. Some tokens (%G, %V, %u, %s) vary by platform — glibc supports them, BSD sometimes doesn't, and Windows lacks many.

What's the difference between %y and %Y?

%Y is the four-digit year; %y is the two-digit year (00–99). Use %Y in any new format string to avoid the Y2K-style ambiguity %y introduces.

How do I include a literal % sign?

Use %% — two percent signs render as one literal %.

Is the tool offline-capable?

Yes. Rendering uses a local strftime implementation in your browser; no network access is required.

%Y %m %d · click to append · live

Visual strftime token builder

Browse every common strftime token. Click to append into the pattern field — output updates live for the current moment.

strftime Builder

Pattern

%Y-%m-%d %H:%M:%S

input

Tokens

available

Output

2026-09-26 17:05:25

now

Mode

live

client

input · tokens

output · preview

Output (now)

2026-09-26 17:05:25

pattern → %Y-%m-%d %H:%M:%S

Quick Start

How strftime works

Every token starts with %. Common ones: %Y year, %m month, %d day, %H 24h, %M minute, %S second. Use %-m to drop the zero pad. %% is a literal percent.

01
Pick a date part
Click any token chip to append it.
Parsing accepts ISO 8601, RFC 3339, RFC 2822, and Unix epoch seconds/milliseconds. The Date constructor handles the rest with browser-engine consistency.
clickISO 8601RFC 3339epochlive
in · timestampclick · ISO
```
+ %Y
```
loaded · ISO 8601zone · ready
02
Add separators
Type spaces, hyphens, colons — anything non-% is literal.
All math runs in UTC internally to avoid DST landmines; presentation converts back to the user’s zone via Intl.DateTimeFormat.
literalUTC mathIntlDST-safelive
raw · optionschar · live
```
%Y-%m-%d
```
options applied · UTCDST-safe
03
Watch output
Output is the result for the current moment.
Output respects the chosen timezone offset (Z vs +05:30) and pads digits per ISO conventions so sorts stay chronological.
liveoffsetzone-awaresortablepadded
now · computedrender · ready
```
2026-05-21 12:00:00
```
computed · instantsortable
04
Copy pattern
One click — drop the pattern into Python, Ruby, C, …
Copy writes via the Async Clipboard API; nothing pings a time server — the browser’s clock is the only oracle.
copyclipboardno oracledeterministicclient-side
✓ · copiedready · done
```
✓ Copied
```
shipped · ready to shipsuccess

Pro tips

Time math without DST landmines

Always store UTC

Convert to local for display only. UTC storage avoids DST jumps and timezone drift in databases.

Watch leap seconds

POSIX time doesn’t count leap seconds; some systems do. Don’t use timestamps for sub-second precision across boundaries.

Prefer Temporal eventually

The Date API has many quirks. Temporal is shipping in modern engines and handles zones correctly.

Cron is timezone-tricky

Cron expressions run in the daemon’s local zone unless told otherwise. Pin a TZ explicitly for predictable runs.

ISO 8601 sorts naturally

YYYY-MM-DDTHH:mm:ss sorts correctly as a string. Don’t use US-style MM/DD/YYYY for anything machine-readable.

Beware Y2K38

Signed 32-bit Unix timestamps overflow in 2038. Use 64-bit storage end-to-end for long-lived systems.

Use Cases

When to author strftime

Logs, scripts, exports.

Log timestamps

Match an existing log format exactly. Math runs in UTC under the hood to dodge DST jumps — local time is only for display. That means scheduling logic and date diffs survive March / November fall-back without the classic off-by-one-hour bug.

DST-safe

Filenames

Build sortable date prefixes for exports. Browser clock is the only source — no external time server, no privacy leak, no rate limit. Pair with NTP-synced server time for high-accuracy use cases, but for human-scale work the browser clock is plenty.

UTC-correct

Python scripts

Test the pattern before pasting into datetime.strftime(). ISO 8601 output sorts naturally as strings, perfect for filenames (report-2026-01-15.pdf) and log lines that need chronological ordering without parsing. Drop into S3 keys for free time-bucketing.

ISO-sortable

shell date

Pipe the pattern straight to `date +`. Pairs cleanly with cron, scheduled tasks, and timestamped log analysis. The tool's output drops directly into APScheduler, Linux cron, or GitHub Actions schedule expressions without re-formatting.

spec-grade

Cron names

Decorate cron job output with readable timestamps. Output respects the chosen timezone offset (Z vs +05:30) so REST APIs and database TIMESTAMP WITH TIME ZONE columns round-trip cleanly. No ambiguity between "noon UTC" and "noon local".

zone-aware

Localisation

%A and %B render in browser locale — sanity-check. Confirms the difference between epoch seconds (10 digits) and milliseconds (13 digits) before you commit code that mis-multiplies by 1000 — a classic bug in cross-language time handling.

i18n

FAQ

Frequently asked questions

Is this POSIX strftime?

Mostly. Some platform-specific tokens (like %V on Linux) aren't included — see the cheatsheet for a fuller list.

Why %-m?

GNU extension to drop the zero-pad. Most Linux strftime accepts it.

Does %Z print the IANA name?

No — it prints the browser's locale-formatted timezone (e.g. EST).

Will my pattern work in Python?

Most yes. Python doesn't accept %-m on Windows — use %#m there.

Is anything sent to a server?

No. Token rendering runs locally.

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