Note: You may find adding eday command to your ~/.bashrc (or equivalent) useful:

#!/bin/bash

function eday { # eday now
 local n=$((($(date +%s%9N)/864)*1000))
 local day=${n:0:-14}; local hour=${n:(-14)}
 echo $day.${hour:0:${1-11}} # $1: precision
}

function d2e { # isodate -> eday
 local n=$((($(date -u --date="$1" +%s%9N)/864)*1000))
 local day=${n:0:-14}; local hour=${n:(-14)}
 echo $day.${hour} | sed 's/\.\?0*$//'
}

function e2d { # eday -> isodate
 local second=$(printf "%f" $(($1*86400)))
 echo $(date -u +"%Y-%m-%dT%H:%M:%S.%N%:z" -d "@$second")
}

# If you're an XFCE user, apt install xfce4-genmon-plugin, then do this.

Alternatives:

function edate { # edate now [YY,DDD H:MM:SS]
 local n=$((($(date +%s%9N)/864)*1000))
 local day=${n:0:-14}; local hour=${n:(-14)}
 printf "%'.0f %s:%s:%s\n" "$day" "${hour:0:1}" "${hour:1:2}" "${hour:3:2}"
}

function edate { # edate now [less precise: Darwin]
 secs=$(date +%s); days=$(($secs/86400))
 hour=$(printf "%05d" "$((($secs-($days*86400))*1000/864))")
 printf "%'.0f %s:%s:%s\n" "$days" "${hour:0:1}" "${hour:1:2}" "${hour:3:2}"
}

function e2d { # eday -> isodate [less precise: RaspberryPi]
 local days=${1%.*}
 [[ $1 != *.* ]] && local fraction=0 || local fraction=${1#*.}
 local second=$((days * 86400 + fraction * 86400 / $((10 ** ${#fraction}))))
 echo $(date -u --iso-8601=seconds -d @"$second")
}

Python:

import time, datetime

def e2d(eday):
    return datetime.datetime.utcfromtimestamp(eday*86400)

def d2e(date):
    return time.mktime(date.utctimetuple())/86400

# Script for days:
# wget edays.py

EXPLANATION 19° dyear : 8″ dmonth : 3′ dweek : 8 (&dweek)day : 3 dhour : 42 dminute : 60.629089 dsecond : 19838.34260 ED

RELATIONS 1° dyear = 1000 days 1″ dmonth = 100 days 1′ dweek = 10 days 1 dday = 1 (&dweek)days 1 dhour = 0.1 days = 8640 seconds 1 dminute = 0.001 days = 86.4 seconds 1 dsecond = 0.00001 days = 0.864 seconds = JD - 2440587.5