convert time between timezones using modern python

tzcvt.py

#! /usr/bin/env python3

from argparse import ArgumentParser, ArgumentDefaultsHelpFormatter
import time
import datetime as DT
import zoneinfo as ZI
import sys

TZFILE = "/etc/timezone"

def get_local_timezone():
    with open(TZFILE) as h:
        return h.read().strip()

GMFMT = "{0}-{1:#02}-{2:#02} {3:#02}:{4:#02}"
    
def gmtime_as_str():
    return GMFMT.format(*time.gmtime())
    
def main():
    ap = ArgumentParser(
        prog="tzcvt",
        description="convert date and time between timezones",
        formatter_class=ArgumentDefaultsHelpFormatter,
    )
    ap.add_argument(
        "-i", "--input_format",
        default="%Y-%m-%d %H:%M",
        metavar="FORMAT",
        help="format for strptime(3) to parse time argument",
    )
    ap.add_argument(
        "-o", "--output_format",
        default="%Y-%m-%d %H:%M:%S %z",
        metavar="FORMAT",
        help="format for strftime(3) to print the result",
    )
    ap.add_argument(
        "-t", "--timezone",
        default="UTC",
        metavar="TZ",
        help="name of timezone to interpret time argument",
    )
    ap.add_argument(
        "-l", "--local_timezone",
        default=get_local_timezone(),
        metavar="TZ",
        help="name of timezone to reinterpret and print result",
    )
    ap.add_argument(
        "time",
        nargs='?',
        default=gmtime_as_str(),
        help="input time",
    )
    opts = ap.parse_args()
    in_datetime = DT.datetime.strptime(opts.time, opts.input_format)
    in_date, in_time = in_datetime.date(), in_datetime.time()
    in_tz = ZI.ZoneInfo(opts.timezone)
    in_combined = DT.datetime.combine(in_date, in_time, in_tz)
    out_datetime = in_combined.astimezone(ZI.ZoneInfo(opts.local_timezone))
    sys.stdout.write(out_datetime.strftime(opts.output_format))
    sys.stdout.write("\n")
    

if __name__ == "__main__":
    main()