It would look like {T345701} is applicable to basically all versions of the library, as the line has been unchanged since it was forked into a library - {b4c8d54d019c861168d684b0ebfa98b25b9ccb6d}
{T347759} created the new release.
Updating MW 1.39 from v4.0.0 to v4.1.1 and MW 1.40 from v4.1.0 to v4.1.1 (same as master) is trivial.
https://github.com/wikimedia/mediawiki-libs-Timestamp/compare/v4.0.0...v4.1.1
https://github.com/wikimedia/mediawiki-libs-Timestamp/compare/v4.1.0...v4.1.1
Updating MW 1.35 from v3.0.0 to v4.1.1~~should be doable, without any relevant breaking changes... So probably the easiest way. #xmpreader depends on it, but the dependancy is pretty loose already...~~isn't doable due to an incompatible PHP version changes in the v4.0.0. A v3.0.1 release could be made, or we don't bother backporting...
https://github.com/wikimedia/mediawiki-libs-Timestamp/compare/v3.0.0...v4.0.0