Simple script to convert between time zones.

tz_convert.py

#! /usr/bin/env python3

"""Simple timezone converter"""

import argparse
import datetime
import re
import sys

TZS = [
    # UTC
    UTC := datetime.timezone.utc,
    
    # Japan
    JST := datetime.timezone(datetime.timedelta(hours=9), name='JST'),

    # USA
    PST := datetime.timezone(datetime.timedelta(hours=-8), name='PST'),
    PDT := datetime.timezone(datetime.timedelta(hours=-7), name='PDT'),
    MST := datetime.timezone(datetime.timedelta(hours=-7), name='MST'),
    MDT := datetime.timezone(datetime.timedelta(hours=-6), name='MDT'),
    CST := datetime.timezone(datetime.timedelta(hours=-6), name='CST'),
    CDT := datetime.timezone(datetime.timedelta(hours=-5), name='CDT'),
    EST := datetime.timezone(datetime.timedelta(hours=-5), name='EST'),
    EDT := datetime.timezone(datetime.timedelta(hours=-4), name='EDT'),

    # Australia
    AWST := datetime.timezone(datetime.timedelta(hours=8), name='AWST'),
    AWDT := datetime.timezone(datetime.timedelta(hours=9), name='AWDT'),
    ACST := datetime.timezone(datetime.timedelta(hours=9, minutes=30), name='ACST'),
    ACDT := datetime.timezone(datetime.timedelta(hours=10, minutes=30), name='ACDT'),
    AEST := datetime.timezone(datetime.timedelta(hours=10), name='AEST'),
    AEDT := datetime.timezone(datetime.timedelta(hours=11), name='AEDT'),
    ]

TIMEZONES = {tz.tzname(datetime.datetime.now()): tz for tz in TZS}

TIME_FORMAT = '%Y-%m-%d %H:%M:%S %Z (UTC%z)'

class ParseError(ValueError):
    """Errors in parsing inputs into valid values"""

    def __init__(self, item, message):
        self.item = item
        self.message = message

def _parse_args():
    def upper(st):
        return st.upper()
    parser = argparse.ArgumentParser(prefix_chars=r'/-@',
        formatter_class=argparse.RawDescriptionHelpFormatter,
        description='Convert times between timezones',
        epilog="""The following characters can be used as separators:
    for date: ./-<space>
    for time: .:<space>
    If <space> is used, the date or time must be quoted.

    In addition to ±HHMM, certain timezone names my be used for timezone.""")
    parser.add_argument('date', help='date (YYYYMMDD)')
    parser.add_argument('time', help='time (HHMMSS)')
    parser.add_argument('timezone', help='timezone (±HHMM / name)', type=upper)
    parser.add_argument('-t', '--target-timezone', metavar='±HHMM', dest='tgt',
        help='timezone to convert to (default JST)', default=JST, type=upper)
    parser.add_argument('-f', '--format', default = TIME_FORMAT,
        help='format for output, as strftime format string')

    args=parser.parse_args()
    
    return args

def _parse_date(date):
    pattern = """
(?P<year>\d{4})
(?P<sep>[ ./-])?
(?P<month>\d?\d)
(?P=sep)?
(?P<day>\d?\d)
"""
    if parsed_date := re.match(pattern, date, re.VERBOSE):
        return list(map(int,parsed_date.group('year', 'month', 'day')))
    else:
        raise ParseError('date', f"couldn't parse {date} into a date")

def _parse_time(time):
    pattern = """
^(?P<hour>\d?\d)
(?P<sep>[ .:-])?
(?P<minute>\d{2})
((?P=sep)?
(?P<second>\d{2}))?$
"""
    if parsed_time := re.match(pattern, time, re.VERBOSE):
        return list(map(int, filter(None,
                        parsed_time.group('hour', 'minute', 'second'))))
    else:
        raise ParseError('time', f"couldn't parse {time} into a time")

def _parse_tz(tz):
    pattern = """
(?P<sign>[+-])
(?P<hour>\d?\d)
(?P<sep>[ .:-])?
(?P<minute>\d{2})
"""
    if parsed_tz := re.match(pattern, tz, re.VERBOSE):
        return parsed_tz.group('sign', 'hour', 'minute')
    else:
        raise ParseError('timezone', f"couldn't parse {tz} into a timezone")

def _make_tz(tz):
    if isinstance(tz, datetime.tzinfo):
        return tz
    timezone = TIMEZONES.get(tz)
    if timezone is None:
        tz_parts = _parse_tz(tz)
        hours, minutes = map(int, tz_parts[1:])
        if tz_parts[0] == '-':
            hours = -hours
        timezone = datetime.timezone(
            datetime.timedelta(hours=hours, minutes=minutes), name='')
    return timezone

def _parse_datetime(date, time, tz):
    parts = _parse_date(date)
    parts.extend(_parse_time(time))
    timezone = _make_tz(tz)
    return datetime.datetime(*parts, tzinfo=timezone)

if __name__ == '__main__':
    args = _parse_args()
    try:
        dt = _parse_datetime(args.date, args.time, args.timezone)
    except ParseError as err:
        sys.exit(f'ERROR: Invalid {err.item}')
    tgt = _make_tz(args.tgt)
    print(dt.astimezone(tgt).strftime(args.format))