Dates and Times Checkpoint

25 min · Intermediate

Fix the mistakes

The slips that cost marks in this module. Spot each one, then check the fix.

Real date code mixes all three ideas: parse text, calculate with durations, and show the result in the right zone. This checkpoint builds a small expiry check and then tests your choices.

Parse at the edge, work in real types

Convert text to dates as soon as it enters the program. Inside, use date, datetime and timedelta only. Turn values back into text at the very end, when you print or store them.

Python

from datetime import date, timedelta
start = date.fromisoformat("2026-11-08")print(start + timedelta(days=30))

Example 1 · Reading a log with bad rows

Python

1from datetime import datetime2
3rows = ["2026-11-08 09:15", "bad", "2026-11-08 10:45"]4stamps = []5for row in rows:6    try:7        stamps.append(datetime.strptime(row, "%Y-%m-%d %H:%M"))8    except ValueError:9        print("skipped:", row)10print(stamps[-1] - stamps[0])

Output

skipped: bad
1:30:00

What this code does

A bad row raises ValueError, which is caught so the loop continues.
The gap between the first and the last valid time is 1 hour 30 minutes.

This keeps string handling in one place, and the rest of the code works with real dates.

Pass the date in

A function that calls now() inside cannot be tested. Give it the current date as a parameter. Production code passes the real date, and tests pass a fixed one.

Python

1from datetime import date2
3def is_expired(expiry, today):4    return today > expiry5
6print(is_expired(date(2026, 12, 8), date(2026, 12, 9)))

Example 2 · Subscription expiry with a passed-in date

Python

1from datetime import date, timedelta2
3def expiry(start_text, days):4    return date.fromisoformat(start_text) + timedelta(days=days)5
6def status(exp, today):7    left = (exp - today).days8    return "expired" if left < 0 else f"{left} days left"9
10exp = expiry("2026-11-08", 30)11print(exp)12print(status(exp, date(2026, 12, 1)))13print(status(exp, date(2026, 12, 20)))

Output

2026-12-08
7 days left
expired

What this code does

expiry parses text once and adds a timedelta.
status receives today as a parameter, so each call can be checked with a fixed date.

The same call gives the same answer every time, so a test never fails by surprise.

Store UTC, show local

Keep moments as aware UTC datetimes. When you show one to a person, convert it with astimezone to their zone. Do exact arithmetic in UTC. Keep naive datetimes out of shared code.

Example 3 · Show a UTC moment in two zones

Python

1from datetime import datetime2from zoneinfo import ZoneInfo3
4event = datetime.fromisoformat("2026-11-08T09:30:00+00:00")5for name in ["Asia/Kolkata", "America/New_York"]:6    print(name, event.astimezone(ZoneInfo(name)).strftime("%d %b %H:%M"))

Output

Asia/Kolkata 08 Nov 15:00
America/New_York 08 Nov 04:30

What this code does

The stored text carries an offset, so the parsed value is already aware.
Each viewer sees the same moment on their own clock.

Finish up

Mark complete & continue

Next: The logging Module and Log Levels

1from datetime import datetime2 3rows = ["2026-11-08 09:15", "bad", "2026-11-08 10:45"]4stamps = []5for row in rows:6 try:7 stamps.append(datetime.strptime(row, "%Y-%m-%d %H:%M"))8 except ValueError:9 print("skipped:", row)10print(stamps[-1] - stamps[0])

1from datetime import date, timedelta2 3def expiry(start_text, days):4 return date.fromisoformat(start_text) + timedelta(days=days)5 6def status(exp, today):7 left = (exp - today).days8 return "expired" if left < 0 else f"{left} days left"9 10exp = expiry("2026-11-08", 30)11print(exp)12print(status(exp, date(2026, 12, 1)))13print(status(exp, date(2026, 12, 20)))

1from datetime import datetime2from zoneinfo import ZoneInfo3 4event = datetime.fromisoformat("2026-11-08T09:30:00+00:00")5for name in ["Asia/Kolkata", "America/New_York"]:6 print(name, event.astimezone(ZoneInfo(name)).strftime("%d %b %H:%M"))