LWG3262 adds:

The result of formatting a std::chrono::duration instance holding a negative value, or of an hh_mm_ss object h for which h.is_negative() is true, is equivalent to the output of the corresponding positive value, with a STATICALLY-WIDEN("-") character sequence placed before the replacement of the leftmost conversion specifier.

[Example:

cout << format("{%:T}", -10'000s); // prints: -02:46:40
cout << format("{:%H:%M:%S}", -10'000s); // prints: -02:46:40
cout << format("{:minutes %M, hours %H, seconds %S}", -10'000s); // prints: minutes -46, hours 02, seconds 40

— end example]

This has a few bugs (fixes already approved on LWG reflector):

cout << format("{%:T}", -10'000s); // prints: -02:46:40

... is wrong. Should be :%T not %:T.

cout << format("{:minutes %M, hours %H, seconds %S}", -10'000s); // prints: minutes -46, hours 02, seconds 40

... is wrong; chrono-specs is required to start with a %. Best we can do here is

cout << format("minutes {:%M, hours %H, seconds %S}", -10'000s); // prints: minutes -46, hours 02, seconds 40

This is misleading:

placed before the replacement of the leftmost conversion specifier.

As noted, the chrono-specs must start with such a conversion specifier. Also,"leftmost" is problematic in an RTL setting. Replace with

placed before the replacement of the leftmost initial conversion specifier.