Dates and Times

Why this matters

Dates look easy until you meet leap years, month lengths and weekends. The datetime module does the hard parts, so you never count days by hand.

The idea

from datetime import date, datetime, timedelta

d = date(2024, 3, 5)                  # year, month, day
d.year, d.month, d.day                # 2024 3 5
d.weekday()                           # 1  (Monday is 0, Sunday is 6)
d + timedelta(days=30)                # date(2024, 4, 4)
date(2024, 12, 25) - d                # timedelta(days=295)
(date(2024, 12, 25) - d).days         # 295

d.strftime("%d %B %Y")                # '05 March 2024'  (date -> text)
datetime.strptime("2024-03-05", "%Y-%m-%d").date()   # text -> date

Common format codes: %Y year, %m month number, %d day, %B month name, %A weekday name, %H:%M hours and minutes. date.today() gives today’s date, but functions that take the date as a parameter are much easier to test.

Try it

Work out how many days there are from today until the end of the year. Then print today’s weekday name with strftime("%A").

Your turn

Write four functions:

• days_until(today, target): the number of days from today to target, both date objects. It is negative when target is in the past.
• format_date(d): text like 05 March 2024.
• parse_date(text): turn "2024-03-05" into a date. Other formats raise ValueError.
• add_business_days(start, n): the date n business days after start, skipping Saturdays and Sundays. From Friday 8 March 2024, one business day later is Monday 11 March.

Recap

• date holds a day, timedelta a length of time; subtract dates to get a timedelta.
• strftime formats a date as text; strptime parses text into a date.
• Pass dates into functions instead of calling date.today() inside, so they can be tested.

Common mistakes

• Mixing up date and datetime: TypeError: can't compare datetime.datetime to datetime.date.
• Wrong format codes (%m is the month number, %M is minutes), which raise ValueError when parsing.
• Forgetting that Monday is 0, so weekdays are 0 to 4.