tztable.py

from zoneinfo import ZoneInfo
from datetime import datetime, timedelta, date, time
import sys


def describe_change(old_list, new_list, names_list):
    diffs = []
    for old, new, name in zip(old_list, new_list, names_list):
        old_tz, old_dst = old
        new_tz, new_dst = new
        if old_tz != new_tz:

            if old_dst.total_seconds() > 0 and new_dst.total_seconds() == 0:
                diffs.append(f"{name} stops observing DST")
            elif old_dst.total_seconds() == 0 and new_dst.total_seconds() > 0:
                diffs.append(f"{name} switches to DST")
            else:
                # non-dst timezone change (rare for our timezones!)
                diffs.append(f"{name} changes: {old_tz - new_tz}")

    print("Changes:")
    for d in diffs:
        print(f"  - {d}")


def parse_args(args):
    result = {
        a.split("=")[0]: int(a.split("=")[1])
        if "=" in a and a.split("=")[1].isnumeric()
        else a.split("=")[1]
        if "=" in a
        else True
        for a in args
        if "--" in a
    }
    result["[]"] = [a for a in args if not a.startswith('--')]
    return result


def print_offsets(offsets):
    printable = []
    for offset, dst in offsets:
        printable.append((str(offset), dst.total_seconds() > 0))

    return str(printable)

def teezee(timezones, start_date, start_hour=5):
    current_timezone = timezones[0]
    time_of_day = datetime.combine(start_date, time(hour=start_hour, minute=0))

    offsets_at_start = [(tz.utcoffset(time_of_day), tz.dst(time_of_day)) for tz in timezones]

    result = {}
    for hours in range(24):
        result[time_of_day.strftime("%H:%M")] = []

        for tz in timezones[1:]:
            result[time_of_day.strftime("%H:%M")].append(time_of_day.astimezone(tz).strftime("%H:%M"))

        time_of_day = time_of_day + timedelta(hours=1)

    for k, v in result.items():
        times = [k]
        times.extend(v)
        print(",".join(times))

    morning = datetime.combine(start_date, time(hour=8, minute=0, second=0).replace(tzinfo=current_timezone))

    for day in range(365):
        morning = morning + timedelta(days=1)
        if offsets_at_start != [(tz.utcoffset(morning), tz.dst(morning)) for tz in timezones]:
            print(f"Valid until {morning.date()}")
            print()
            describe_change(offsets_at_start, [(tz.utcoffset(morning), tz.dst(morning)) for tz in timezones], timezones)
            break

    return morning.date()


if __name__ == "__main__":
    timezones = [
        ZoneInfo("Australia/Melbourne"),
        ZoneInfo("America/Los_Angeles"),
    ]

    args = parse_args(sys.argv[1:])

    if args["[]"]:
        print([int(x) for x in args["[]"][0].split("-")])
        d = date(*[int(x) for x in args["[]"][0].split("-")])
    else:
        d = date.today()

    while True:
        print(",".join(["Melbourne", "San Francisco"]))
        d = teezee(timezones, d)
        i = input("---")
        if i != "":
            break