fix(date): `date --date=` flag returns empty output

[chaliy]

Summary

`date +FORMAT --date=STRING` returns empty output instead of parsing the given date string.

Reproduction

date +"%B %d, %Y" --date="Mon, 06 Apr 2026 12:00:00 +0000"
# Expected: April 06, 2026
# Actual: (empty string)

Context

Discovered while running bashblog via bashkit CLI. Bashblog uses `--date=` in `create_html_page` to format timestamps stored in blog posts:

echo "<!-- $date_inpost: #$(LC_ALL=$date_locale date +"$date_format_timestamp" --date="$timestamp")# -->"
echo -n "<div class=\"subtitle\">$(LC_ALL=$date_locale date +"$date_format" --date="$timestamp")"

Without this, all post timestamps and subtitle dates render as empty, producing `` and empty `

` elements.

Expected behavior

`date --date=STRING +FORMAT` should parse the date string and format it according to the format specifier. Should support at minimum RFC 2822 date strings.