The uu_date command in coreutils uses chrono to parse the format string. In #7334 there is a report that it is needed to add the amount of padding to the format string.

current behavior

let format_items = StrftimeItems::new("+%03d"); will yield Item::Error.

expected behavior

let format_items = StrftimeItems::new("+%03d"); should yield Item::Numeric(Numeric::Day, Pad::Zero(3)).
And in format_numeric() replace the hard coded 1 char padding to the value in Pad::Space or Pad::Zero.