CLDR ss

Second, padded.

05

at the reference instant, Mon, Jan 2 2006, 3:04:05 PM MST

The second 00–59, zero-padded (60 possible where leap seconds surface). strftime %S, CLDR ss.

LanguageTokenCaveat
C / POSIX strftime%SRange is [00,60] — 60 allows for leap seconds.
C# custom formatssA bare s (as "%s") gives the unpadded second.
CLDR / ICU patternssA single s prints minimum digits (unpadded). Truncated, not rounded.
Go time layout05A bare 5 gives the unpadded second.
Java DateTimeFormatterss
JavaScript Intl.DateTimeFormatsecond: "2-digit"second: "numeric" exists but many locales render it two-digit anyway.
Moment.js formatssA single s gives the unpadded second.
PHP date()s
Python strftime%S
Ruby strftime%S
Rust chrono%SRange is 00–60 because chrono accounts for leap seconds.

Definition anchored to Unicode UTS #35 — Date Field Symbol Table.