strftime

Format codes for date / time string formatting. The same codes work across most languages’ formatters: C strftime, Python datetime.strftime, Go time.Format (with translation), Java DateTimeFormatter, Ruby strftime, JS via libraries.

Most-Used Codes

The codes that appear in nearly every date format string an operator writes. Year / month / day / hour / minute / second plus weekday and timezone, enough to build any of the standard datetime patterns from scratch.

Code	Example	Description
%Y	2026	4-digit year
%y	26	2-digit year
%m	04	month (01-12)
%d	27	day of month (01-31)
%H	14	hour, 24-hour (00-23)
%I	02	hour, 12-hour (01-12)
%M	05	minute (00-59)
%S	30	second (00-59)
%p	PM	AM / PM
%a	Mon	abbreviated weekday
%A	Monday	full weekday
%b	Apr	abbreviated month
%B	April	full month
%j	117	day of year (001-366)
%w	1	day of week (0=Sunday)
%u	1	ISO day of week (1=Monday)
%U	17	week of year (Sunday-first)
%W	17	week of year (Monday-first)
%V	18	ISO week (01-53)
%G	2026	ISO week-numbering year
%Z	UTC	time-zone name
%z	+0000	time-zone offset
%s	1745765130	Unix timestamp (GNU)

Common Patterns

The format strings that come up over and over. ISO 8601 / RFC 3339 for new APIs and logs; RFC 2822 for email; the compact %Y%m%dT%H%M%SZ for filenames you want sorted lexicographically; %H:%M:%S for time-only displays.

Pattern	Output
%Y-%m-%d	2026-04-27 (ISO date)
%Y-%m-%dT%H:%M:%S	2026-04-27T14:05:30 (ISO datetime; no zone)
%Y-%m-%dT%H:%M:%S%z	2026-04-27T14:05:30+0000 (ISO with offset)
%Y-%m-%d %H:%M:%S %Z	2026-04-27 14:05:30 UTC
%a, %d %b %Y %H:%M:%S %z	Mon, 27 Apr 2026 14:05:30 +0000 (RFC 2822 / email)
%H:%M:%S	14:05:30 (time only)
%Y%m%d	20260427 (compact date; filenames)
%Y%m%dT%H%M%SZ	20260427T140530Z (compact UTC; filenames)
%Y-W%V-%u	2026-W18-1 (ISO week date)

Locale-Sensitive Codes

The codes whose output changes by machine. Avoid them in logs, wire formats, and any tool you might run on a system whose locale you don’t control. %c / %x / %X are the headline offenders here.

Code	Example (en_US)	Description
%c	Mon Apr 27 14:05:30 2026	locale’s date+time
%x	04/27/26	locale’s date
%X	14:05:30	locale’s time

Avoid these in tools / logs / wire formats; the output changes by machine. Use explicit codes.

Less-Common Codes

The codes that come up rarely but read awkwardly when you don’t recognize them. %C is “century”; %e / %k / %l are the space-padded variants of day / hour; %n / %t / %% are the literal escapes.

Code	Example	Description
%C	20	century
%e	27	day-of-month, space-padded (” 1”-“31”)
%k	14	hour, 24-h, space-padded
%l	2	hour, 12-h, space-padded
%n	(newline)	literal newline
%t	(tab)	literal tab
%%	%	literal percent sign

GNU Extensions (Linux strftime)

Extensions GNU added on top of POSIX strftime. %s (epoch seconds) is the most-used; %F / %T / %R are short aliases for the most common ISO patterns. None are portable to non-GNU systems; check before using on macOS or BSD.

Code	Example	Description
%s	1745765130	Unix epoch seconds
%N	123456789	nanoseconds (date / GNU coreutils)
%P	pm	lowercase AM/PM
%R	14:05	short form of %H:%M
%T	14:05:30	short form of %H:%M:%S
%F	2026-04-27	short form of %Y-%m-%d
%D	04/27/26	short form of %m/%d/%y
%r	02:05:30 PM	locale’s 12-hour time

Practical Snippets

Working examples in each language an operator is likely to hit. Bash and Python use familiar strftime codes; JavaScript needs a library or Intl.DateTimeFormat; Go uses its idiosyncratic reference-time pattern instead.

Bash / coreutils

$ date
$ date -u
$ date -Iseconds
$ date +%s
$ date +%Y-%m-%dT%H:%M:%SZ -u
$ date +%Y%m%dT%H%M%SZ -u

$ date -d "2026-04-27 14:05" +%s

$ date -j -f "%Y-%m-%d" "2026-04-27" +%s

Python

from datetime import datetime, timezone

datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")
# 2026-04-27T14:05:30Z

datetime.strptime("2026-04-27 14:05", "%Y-%m-%d %H:%M")

# ISO format helper
datetime.now(timezone.utc).isoformat()
# 2026-04-27T14:05:30+00:00

JavaScript

JS lacks strftime; use Intl.DateTimeFormat or libraries.

// ISO
new Date().toISOString()
// 2026-04-27T14:05:30.123Z

// Locale-aware
new Intl.DateTimeFormat("en-US", { dateStyle: "medium", timeStyle: "short" })
  .format(new Date());

// date-fns / dayjs / luxon for token-based formatting
format(new Date(), "yyyy-MM-dd HH:mm:ss")    // date-fns
dayjs().format("YYYY-MM-DD HH:mm:ss")        // dayjs

Go

Go uses a reference time instead of strftime codes. Memorize one example: Mon Jan 2 15:04:05 MST 2006 (or 01/02 03:04:05PM '06 -0700).

import "time"

time.Now().UTC().Format(time.RFC3339)
// 2026-04-27T14:05:30Z

time.Now().Format("2006-01-02 15:04:05")
// 2026-04-27 14:05:30

Common Pitfalls

The mistakes that break date handling silently in production. %M vs %m is the most-confused pair; %s and %V / %G have portability and pairing footguns; locale-sensitive codes mangle output the moment a script lands on a non-en_US host.

• %M (minute) vs. %m (month), the most-confused pair.

• %y (2-digit year) is rarely what you want.

• %Z may be empty depending on locale / system; %z (offset) is more reliable.

• %s (epoch seconds) is GNU-only; not in POSIX.

• %V / %G (ISO week / week-year), use both together; never %V with %Y.

• JS doesn’t have strftime natively; libraries differ in token syntax.

• Avoid locale-dependent codes (%c, %x, %X, %a, %A, %b, %B) in logs and wire formats.

Standards Worth Remembering

The five datetime standards that operators encounter in the wild. ISO 8601 / RFC 3339 is the modern default for new APIs; the RFC 2822 / 5322 / HTTP-date formats appear in protocol headers; the Unix epoch is the wire format for almost every machine-to- machine timestamp.

• ISO 8601, YYYY-MM-DDTHH:MM:SS±hh:mm (or with Z for UTC).

• RFC 3339, a profile of ISO 8601 used by JSON / OpenAPI / most web APIs.

• RFC 2822 / 5322, email format: Mon, 27 Apr 2026 14:05:30 -0500.

• HTTP-date (RFC 9110), Mon, 27 Apr 2026 14:05:30 GMT.

• Unix epoch, seconds since 1970-01-01T00:00:00Z.

For new APIs and logs, prefer ISO 8601 / RFC 3339 with explicit time zones.