Cron Expression Next-Run Calculator

#!/usr/bin/env python3
"""Calculate the next run times for a cron expression.

Dependency: pip install croniter
"""
import argparse
from datetime import datetime, timezone

from croniter import croniter, CroniterBadCronError


def next_runs(expression, count, base):
    """Return a list of the next `count` datetimes after `base`."""
    itr = croniter(expression, base)
    return [itr.get_next(datetime) for _ in range(count)]


def main():
    parser = argparse.ArgumentParser(
        description="Compute upcoming run times for a cron expression.")
    parser.add_argument("expression",
                        help='Cron expression, e.g. "*/15 * * * *".')
    parser.add_argument("-n", "--count", type=int, default=5,
                        help="How many upcoming runs to show (default: 5).")
    parser.add_argument("--utc", action="store_true",
                        help="Use UTC instead of local time.")
    args = parser.parse_args()

    base = datetime.now(timezone.utc) if args.utc else datetime.now()

    # Validate the expression before computing anything.
    if not croniter.is_valid(args.expression):
        print(f"Invalid cron expression: {args.expression}")
        return

    try:
        runs = next_runs(args.expression, args.count, base)
    except CroniterBadCronError as exc:
        print(f"Error: {exc}")
        return

    tz_label = "UTC" if args.utc else "local"
    print(f"Cron: {args.expression}")
    print(f"From: {base.strftime('%Y-%m-%d %H:%M:%S')} ({tz_label})\n")
    for i, run in enumerate(runs, start=1):
        delta = run - base
        print(f"{i:>2}. {run.strftime('%Y-%m-%d %H:%M:%S')}  "
              f"(in {str(delta).split('.')[0]})")


if __name__ == "__main__":
    main()