You need to download the translation file .po (Not .pot) from Module Statistics: nautilus for the version and language you want. Then open it with a translation editor or a text editor to change the respective line, then compile the .po file to .mo with gettext (google it), then copy it to the installation place, something like /usr/share/locale/en/LC_MESSAGES/ or /usr/share/locale-langpack/en/LC_MESSAGES/
This is because there’s only one default date and time formats but the entire feature hinges upon having different levels of details. Neither Glibc or GLib can provide different lengths or variants of date/time formats and thus the strings has to be translated manually. The proper solution that requires less manual translation is to bring an ICU dependency, but that’s unlikely to be added just to support the translation of a few strings.