Bash function to display a given time in multiple timezones

tz.bash

#!/bin/bash
# tz  <TIME SPECIFICATION>  
#
# Display the specified time in multiple timezones.
#
# If no time is specified then "now" is used.
#
# You may want to customize the ZONES array below with your 
# favorite timezone names (see also tzselect)   
#
# Timezones are automatically sorted by their UTC offsets.
#
# Timezones with the same UTC offset than the local timezone
# are marked with '*'
#
# However, they are marked with '?' instead if the UTC offset
# for the current time and for the specified time are not equal.
# That typically indicates a summer/winter time change between
# the two dates.
#
# See 'info date' for a full description of the supported time
# and date formats.
#
# Examples: 
#
#   tz                    : for the current time (or "now")  
#   tz 14:00              : for 14:00 today in the current timezone
#   tz 14:00+4            : for 14:00 today in UTC+4 
#   tz tomorrow 2pm PST   : for 14:00 tomorrow in Pacific Standard Time 
#   tz 2pm July 3 CEST    : for Friday July 3 14:00 2015 in Central European Summer Time 
#
# Remark: Not all symbolic timezone names work as expected.
#         First because some of them are only valid during parts of
#         the year (e.g CET vs CEST in Central Europe) and also because
#         some names are ambiguous (e.g. CST may refer to Central Standard
#         Time in the USA or to the China Standard Time in China).
#         Using UTC or GMT relative timezones is usually safer.
#
# http://www.timeanddate.com/time/map/
#
tz () {
    local FMT A D T tz M UTC0 UTC1 UTC2 ZONES 
    D="$*"
    # Output format (see man 1 date)
    FMT="%H:%M %_4Z (UTC%:z) %a %b %d"
    # FMT="%c %Z (UTC%:z)"
    [ -n "$D" ] || D=now    
    T=$(date -d "$D" +%s) || return
    # Add here your preferred locations (as given by tzselect)
    ZONES=(        
        America/Los_Angeles   # PST/PDT
        # America/Denver       # MST/MDT
        America/Chicago       # CST/CDT
        America/New_York      # EST/EDT
        Europe/London         # GMT/BST
        Europe/Paris          # CET/CEST 
        # Asia/Shanghai         # CST (China Standard Time)
        Asia/Tokyo            # JST 
        # Australia/Melbourne   # AEDT/AEST
        # Australia/Adelaide    # ACDT/ACST
    )
    # Get current timezone now (in $UTC0) and at requested time (in $UTC1)
    # They may be different because of summer/winter time changes. 
    UTC0=$(LC_ALL=C  date -d "now" +"%z")
    UTC1=$(LC_ALL=C  date -d "@$T" +"%z")
    for tz in "${ZONES[@]}" ; do
        # UTC2 is the UTC timezone of $tz at the given time 
        UTC2=$(LC_ALL=C TZ="$tz" date -d "@$T" +"%z")
        XX=$?
        if [[ "$UTC1" == "$UTC2" ]] ; then
            if [[ "$UTC0" == "$UTC1" ]] ; then
                M="*"  # This is consistent with current timezone.
            else
                M="?"  # Also but probably with a summer/winter time change.
            fi
        else
            M=" "
        fi
        A=$(LC_ALL=C TZ="$tz" date -d "@$T" +"$FMT")
        # Prefix each output line with its numeric timezone. 
        # That prefix will we removed after sorting.
        printf "%s %c%-20s %s $XX\n" "$UTC2" "$M" "$tz" "$A"
    done | sort -n | cut -c 7-
}

tz "$@"