macOS ISO8601 dates

my_date.sh

my_date () {
  local date_cmd
  local date

  # Check if GNU date installed as gdate
  : type gdate 2> /dev/null
  [ $? == 0 ] && date_cmd=gdate || date_cmd=date
  
  # Attempt GNU date - date will be unset if this fails
  date=$(${date_cmd} --iso-8601=second 2> /dev/null)
  # Use date if unset. This will hopefully be BSD date
  echo ${date:=$(local x=$(date '+%FT%T%z'); echo ${x/%00/}):00 }
}