cron - work out when a cron expression will next run (or last ran)

Usage:
  curl --data "<expression>" https://ttykit.com/cron
  curl --data "<expression>" "https://ttykit.com/cron?n=10"
  curl --data "<expression>" "https://ttykit.com/cron?history=1"

  (cron expressions contain spaces and '*', which most shells/curl won't
  pass through unencoded in a URL, so POSTing the expression as the body
  is the easiest way in. If you'd rather use a query param, percent-encode
  it or let curl do that for you:)
  curl -G https://ttykit.com/cron --data-urlencode "expr=0 9 * * MON-FRI"

Standard 5-field cron syntax: minute hour day-of-month month day-of-week
  *  any value        */5     every 5 units
  1,15,30              list   1-5     range   1-10/2  range with step
  JAN-DEC and SUN-SAT names are accepted for month/day-of-week
  Shortcuts: @yearly @monthly @weekly @daily @hourly

If day-of-month and day-of-week are both restricted (not '*'), a run
matches when EITHER one hits, matching standard cron/Vixie-cron behavior.

All times are UTC.
  n=         how many runs to list (default 5, max 50)
  history=1  look backward instead of forward: list the most recent past
             runs (oldest to newest) instead of upcoming ones

Examples:
  curl --data "0 9 * * MON-FRI" https://ttykit.com/cron
  curl --data "*/15 * * * *" "https://ttykit.com/cron?n=8"
  curl --data "@daily" https://ttykit.com/cron
  curl --data "0 9 * * MON-FRI" "https://ttykit.com/cron?history=1&n=10"

Crontab file mode (POST a crontab, one schedule per line, '#' for comments,
optional command/label after the 5 fields or shortcut). Real /etc/crontab
files work directly: VAR=value environment lines (SHELL=, PATH=, MAILTO=,
etc.) are skipped just like the real cron daemon does.
  curl --data-binary @crontab.txt https://ttykit.com/cron/file
  curl --data-binary @/etc/crontab "https://ttykit.com/cron/file?n=3"
  curl --data-binary @crontab.txt "https://ttykit.com/cron/file?history=1"

Got an /etc/anacrontab instead of a crontab? That's a different format
(period/delay/job-id, not minute-hour-dom-month-dow) -- see ttykit.com/cron/anacron