days - print the number of days between now and a given date

days.py

#!/usr/local/bin/python3
"""\
Days
    Calculate the number of days between a date and today or another date

Usage:
    days <date> [<second_date> --format=<fmt>]

Options:
    -f <fmt>, --format=<fmt>    A strptime format string
                                [default: %Y-%m-%d]
"""

import sys
from docopt import docopt
from datetime import datetime


def parse_date(date_string, date_format):
    """datetime.strptime wrapper which exits on error

    Attempt to parse date_string with date_format, both of which are
    given by the user. On error, print the reason to stderror and exit
    """
    try:
        date = datetime.strptime(date_string, date_format)
    except ValueError as error:
        sys.exit(error)
    else:
        return date

args = docopt(__doc__)

if args['<second_date>']:
    start_date = parse_date(args['<second_date>'], args['--format'])
else:
    start_date = datetime.today()

target_date = parse_date(args['<date>'], args['--format'])
difference = (target_date.date() - start_date.date()).days
# .date() used to zero the time, otherwise you get odd results

if difference == 0 and not args['<second_date>']:
    print("That's today!")
else:
    days = 'day' if difference in [1, -1] else 'days'
    message = '{} {} '.format(abs(difference), days)
    if args['<second_date>']:
        message += 'apart'
    else:
        message += ('ago' if difference < 0 else 'ahead')
    print(message)

days.sh

#!/bin/bash

# Assign first argument or stdin to $DATE
if [ -t 0 ]; then
    DATE=$1
else
    read DATE
fi

if [ "$DATE" == "" ]; then
    # Print usage message if no date is supplied
    echo "Usage:  $(basename $0) YYYY-MM-DD"
    echo "        prints number of days until or since the given date"
    exit 64
fi

# Exit if $DATE is not in ISO format
echo $DATE | egrep "[0-9]{4}-[0-9]{2}-[0-9]{2}" > /dev/null
if [ $? -ne 0 ]; then
    echo "Abort:  your date is not in YYYY-MM-DD format"
    exit 65
fi

THEN=$(date -jf %Y-%m-%d $DATE +%s)
NOW=$(date -j +%s)
DAYS=$(bc <<< "($THEN - $NOW) / 86400")

if [ $DAYS -eq 0 ]; then
    echo "That's today!"
    exit 0
fi

MSG="$DAYS day"
if [ $DAYS -ne 1 ] && [ $DAYS -ne -1 ]; then
    # Plural if number of days is not 1 and not -1
    MSG="${MSG}s"
fi

if [ $DAYS -lt 0 ]; then
    # Trim sign if a negative number
    echo $(cut -c 2- <<< $MSG) ago
else
    echo $MSG ahead
fi
exit 0

This is handy. Is there a way to format the output in larger units, if it's a date in the far future? So, maybe, X years, Y months, Z days ahead?

@crmdgn (Sorry, only just seen your comment — no notifications for gists.)

Days are the largest unit stored by Python's datetime.timedelta objects (see line 40), and once you start dealing in months and years you've got to consider that they vary in length. It is possible, of course, but would require some thought to provide an answer that uses units larger than a week. At that point you're in the realm of true calendrical calculations as opposed to just subtracting one integer from another.