How to subtract a day from a date?

Time zones and daylight saving

Earth isn't flat, mind time zones! We live in a time of global reach. If your application crosses the borders, consider tzinfo from the datetime module, or better yet, third-party libraries like pytz or pendulum.

Keeping time: subtracting a day in 'US/Eastern':

import pytz
from datetime import datetime, timedelta

# "What's today?" "Why, it's Christmas day, sir!"
today = datetime.now(pytz.timezone('US/Eastern'))

# Subtract a day whilst saying "Bah, humbug!" to DST and time zone errors
yesterday = today - timedelta(days=1)
yesterday = yesterday.astimezone(pytz.timezone('US/Eastern'))

print(yesterday)  # Ebenezer Scrooge approves.
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Going granular with relativedelta

Crying over spilt seconds? When every millisecond counts, dateutil.relativedelta offers precision for custom time calculations.

Subtracting a day, end-of-the-month headaches notwithstanding:

from datetime import datetime
from dateutil.relativedelta import relativedelta

# "Be kind whenever possible. It is always possible."
today = datetime.now()
yesterday = today - relativedelta(days=1)

print(yesterday)  # Yesterday brought the beginning, tomorrow brings the end.
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Winning reliability points

How to not lose at date manipulation:

• Don't play hide and seek: Always handle time zones explicitly.
• If you're picky with your time zones, go for tzlocal.get_localzone to get by.
• Time travelling? Research the DST rules for historical dates.

Advanced cases

A date with pendulum

Pendulum to the rescue: no more DST and timezone adjustments.

import pendulum

today = pendulum.now('US/Eastern')
# Bye to today, hello yesterday!  
yesterday = today.subtract(days=1)
print(yesterday)
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Negative timedelta for the win

Just because we can't turn back time (except in songs ⏪🎵), doesn't mean our code can't!

# Who said we can't time travel? Back to the future!
last_week = today + timedelta(weeks=-1)
print(last_week)  # Marty McFly: "Whoa, this is heavy."
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Chasing the masses

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