strftime / Date Format String Builder

strftime ("string format time") comes from the C standard library and is used almost unchanged by Python's datetime.strftime, Ruby's Time#strftime, PHP's deprecated strftime(), Perl's POSIX module and the Unix date +FORMAT command. A format string mixes literal text with directives that start with %: %Y-%m-%d becomes 2026-03-05 and %A, %B %e becomes Thursday, March 5.

The preview uses the C locale (English names, %c as Thu Mar 5 14:07:09 2026). The date and time you enter are used as-is, and the UTC offset menu supplies %z and %Z, so the preview never depends on your computer's time zone. Directives that are not defined, such as %Q, are shown unchanged and flagged; Python passes unknown directives through on most platforms, but some C libraries print nothing or crash.

Other ecosystems use different systems, and the tool converts the whole string for you:

Go formats by example: you write the reference time Mon Jan 2 15:04:05 MST 2006 the way you want it to look. Fractional seconds must follow a period (05.000000), and Go has no layout for weekday numbers or week-of-year, so those are flagged.

PHP uses single letters, so literal letters must be escaped with a backslash; the translation adds the escapes (at becomes \a\t).

date-fns, Java's DateTimeFormatter, Moment's newer tokens and ICU all follow Unicode LDML patterns, where literal text is wrapped in single quotes and YYYY means week-based year, a common source of bugs in late December. The translation always uses yyyy. date-fns also refuses DDD (day of year) unless you pass { useAdditionalDayOfYearTokens: true } to format.