date doesn't allow format strings that are not UTF-8

GNU:

```
$ LC_ALL=en_US.ISO-8859-1 date +$(LC_ALL=en_US.ISO-8859-1 printf '\xC2') | iconv --from-code=ISO-8859-1 --to-code=UTF-8
Â
```
uutils:

```
$ LC_ALL=en_US.ISO-8859-1 ./target/debug/date +$(LC_ALL=en_US.ISO-8859-1 printf '\xC2') | iconv --from-code=ISO-8859-1 --to-code=UTF-8
error: invalid UTF-8 was detected in one or more arguments

For more information, try '--help'.
```