datediff - count the days between two dates

Usage:
  curl https://ttykit.com/datediff/<date1>/<date2>
  curl "https://ttykit.com/datediff/<date1>/<date2>?exclude_weekends=1&exclude_holidays=1&holidays=2026-12-25"

  curl --data-binary @dates.txt https://ttykit.com/datediff/file
  curl --data-binary @dates.txt "https://ttykit.com/datediff/file?exclude_weekends=1"

Dates accept: YYYY-MM-DD, MM/DD/YYYY, MMDDYY, MMDDYYYY, "Jan 5 2026", today, yesterday, tomorrow

Query params:
  exclude_weekends=1   don't count Saturdays/Sundays
  exclude_holidays=1   don't count US federal holidays (built-in calendar, observed dates)
  holidays=D1,D2,...   comma-separated extra dates to exclude on top of that (any accepted format)

Examples:
  curl https://ttykit.com/datediff/2026-01-01/today
  curl https://ttykit.com/datediff/today/tomorrow
  curl "https://ttykit.com/datediff/2026-01-01/2026-03-15?exclude_weekends=1"

File mode (POST a body, one date per line, '#' for comments):
  echo "2026-01-15 | Project kickoff
  2026-03-01 | Beta launch
  2026-06-01 | GA release" | curl --data-binary @- https://ttykit.com/datediff/file

Dates are sorted and diffed consecutively; a total is printed at the end.
Label a date with '| text' after it (shown back in the output).