Add business days while skipping weekends

By Batu · English technical notes

Also published in our Blogger archive.

“Three business days after Friday” is not three calendar days after Friday. This example defines business days as Monday through Friday, advances one timedelta(days=1) at a time, and decrements the remaining count only when date.weekday() is below 5. Starting on Friday 2026-05-01, it visits Saturday and Sunday without counting them, then counts Monday, Tuesday, and Wednesday. The returned date is 2026-05-06.

The function deliberately accepts only non-negative counts. Its zero case returns the starting date because the loop does not run. That choice should be stated in a caller’s contract: some systems instead define “zero business days” as the next business day when the input is a weekend. The assertions show the selected convention for this one Friday input and do not prove correctness for every calendar.

Weekends are not a complete business calendar. This code neither knows public holidays nor short working days, regional weekends, closures, or deadlines with times of day. Add those rules from a maintained, jurisdiction-specific source if they matter. The example uses only date, timedelta, and date.weekday() from Python’s standard library and works on Python 3.2+.

The Python datetime documentation defines date arithmetic and documents that weekday() maps Monday to 0 and Sunday to 6.

AI-assistance disclosure: This article was drafted with AI assistance and should be reviewed in its application context.

from datetime import date, timedelta


def add_business_days(start, count):
    if count < 0:
        raise ValueError("count must be non-negative")

    current = start
    remaining = count
    while remaining:
        current += timedelta(days=1)
        if current.weekday() < 5:
            remaining -= 1
    return current

arrival = add_business_days(date(2026, 5, 1), 3)

assert arrival == date(2026, 5, 6)
assert arrival.weekday() == 2

print(arrival.isoformat())

2026-05-06