Moved from bug 35646 comment 17. Tt doesn't work because we don't set Locale for the DateTime objects we create in Koha::DateUtils: https://metacpan.org/pod/DateTime#Locales We could easily pass through the 'locale' from the Serial (subscription table) however, I spot tested and got an error for 'Ethiopic'/'gez' though it is listed here: https://metacpan.org/pod/DateTime::Locale::Catalog Our list comes from here: https://www.loc.gov/standards/iso639-2/php/code_list.php Maybe our datetime modules need an update? Here is a full list of ones that throw an error: Invalid locale code or name: akk Invalid locale code or name: tut Invalid locale code or name: grc Invalid locale code or name: ave Invalid locale code or name: bnt Invalid locale code or name: bra Invalid locale code or name: cel Invalid locale code or name: chp Invalid locale code or name: cho Invalid locale code or name: chu Invalid locale code or name: syc Invalid locale code or name: cop Invalid locale code or name: cre Invalid locale code or name: prs Invalid locale code or name: dgr Invalid locale code or name: egy Invalid locale code or name: gez Invalid locale code or name: gem Invalid locale code or name: got Invalid locale code or name: gwi Invalid locale code or name: her Invalid locale code or name: hit Invalid locale code or name: inc Invalid locale code or name: iku Invalid locale code or name: ipk Invalid locale code or name: ira Invalid locale code or name: jrb Invalid locale code or name: csb Invalid locale code or name: lad Invalid locale code or name: lat Invalid locale code or name: myn Invalid locale code or name: dum Invalid locale code or name: enm Invalid locale code or name: frm Invalid locale code or name: gmh Invalid locale code or name: mga Invalid locale code or name: mul Invalid locale code or name: nah Invalid locale code or name: zxx Invalid locale code or name: nya Invalid locale code or name: arc Invalid locale code or name: ang Invalid locale code or name: fro Invalid locale code or name: goh Invalid locale code or name: peo Invalid locale code or name: pro Invalid locale code or name: ota Invalid locale code or name: pal Invalid locale code or name: pli Invalid locale code or name: pbr Invalid locale code or name: pra Invalid locale code or name: roa Invalid locale code or name: rom Invalid locale code or name: sam Invalid locale code or name: sem Invalid locale code or name: den Invalid locale code or name: sla Invalid locale code or name: wen Invalid locale code or name: sux Invalid locale code or name: syr Invalid locale code or name: tgl Invalid locale code or name: tet Invalid locale code or name: uga Invalid locale code or name: mis