The datetime module has date for calendar days, datetime for a day plus a time of day, and timedelta for a length of time.

from datetime import date, timedelta

d = date.fromisoformat("2024-02-27")
later = d + timedelta(days=3)
print(later)               # 2024-03-01
print((later - d).days)    # 3
print(later.strftime("%d %b %Y"))
print(later.weekday())     # 4, a Friday
print(later.isoformat() > "2024-01-31")

• fromisoformat reads, and isoformat writes, the YYYY-MM-DD form. Text in that form also sorts in date order.
• Adding a timedelta handles month ends and leap years for you. Subtracting two dates gives a timedelta, and .days is a whole number.
• strftime formats a date with codes such as %Y, %m, %d and %A, the weekday name. strptime parses text with the same codes.
• weekday() counts Monday as 0.

date.today() and datetime.now() change every day. Pass dates into your functions instead of reading the clock inside them, so the functions stay testable.

Write days_between(start, end) taking two YYYY-MM-DD strings and returning the number of days from start to end, negative if end is earlier. Write due_date(start, days) returning the date days after start, as a YYYY-MM-DD string.

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