Show the current Decimal time or convert from "Standard time" to Decimal time

dtime.sh

#!/bin/sh -eu
# Check for -s as an argument and if so convert decimal time to 'standard time'
if [ "${1-}" = '-s' ]; then
  if [ -n "${2-}" ]; then
    TIME_IN_SECS="$(echo 864*$(echo "$2" | tr -d :)/10 | bc)"
    if [ "$(uname -s)" = Darwin ]; then
      date -j -f "%s" "$(expr $(date -j -f '%H:%M:%S' '00:00:00' '+%s') + $TIME_IN_SECS)" '+%H:%M'
      exit 0
    else
      date --date="0 today + $TIME_IN_SECS seconds" '+%H:%M'
      exit 0
    fi
  else
    echo "Provide a decimal time to convert" >&2
    exit 1
  fi
fi

# Check for -u as an argument and if so print the time for the IERS Reference Meridian (UTC equivalent)
if [ "${1-}" = '-u' ]; then
  TIME_INPUT="$(date -u '+%H:%M:%S')"
  shift 1
else
  TIME_INPUT="$(date '+%H:%M:%S')"
fi

# Normalise any user provided time formatting provided
if [ -n "${1-}" ]; then
  if echo "$1" | grep -q ':.*:' ; then
    TIME_INPUT="$1"
  elif echo "$1" | grep -q '[0-9]' ; then
    TIME_INPUT="${1}:00"
  fi
fi

# Convert 'standard time' to decimal time
CURRENT_MINSEC="${TIME_INPUT#*:}"
echo "(("${TIME_INPUT%%:*}" * 3600) + ("${CURRENT_MINSEC%:*}" * 60) + "${CURRENT_MINSEC#*:}") / 86.4" \
  | bc | sed 's/^/00/' | tail -c4 | sed 's/\(.*\)\(..\)$/\1:\2/'