#!/usr/bin/env sh
# compat: +ash +bash +dash +hush -ksh +mksh +oksh +osh +posh +yash +zsh

now() { ### @-
    ### print a date-time (UTC) in a sortable format.
    ### this takes a date or file as an argument,
    ### else it defaults to the current time.
    ### ```
    ### $ now
    ### 2019-05-27_35083906
    ### $ now ~/sh/monitor
    ### 2017-03-14_82387259
    ### $ now '@1234567890'
    ### 2009-02-13_84690000
    ### ```
    [ $# -le 1 ] || { printf "%s\n" "$0: too many arguments" >&2; return 1; }

    local dt= fmt='+%F_%T_%N'
    if [ -z "$1" ]; then
        dt="$(exec /usr/bin/env date -u "$fmt")"
    elif [ -e "$1" ]; then
        dt="$(exec /usr/bin/env date -u -r "$1" "$fmt")"
    else
        dt="$(exec /usr/bin/env date -u -d "$1" "$fmt")"
    fi

    local F= T= N=
    F=${dt%%_*}
    T=${dt#*_}
    T=${T%_*}
    N=${dt##*_}
    local H= M= S=
    H=${T%%:*}
    M=${T#*:}
    M=${M%:*}
    S=${T##*:}

    # don't interpret numbers as octal
    F=${F#0*}
    T=${T#0*}
    N=${N#0*}
    H=${H#0*}
    M=${M#0*}
    S=${S#0*}

    local ms=
    ms=$(( (H*60*60+M*60+S)*1000+N/1000000 ))
    ms=$(printf '%08i' $ms)
    echo "${F}_${ms}"
}

[ -n "${preload+-}" ] || now "$@"