Working with Datetime

Python
from datetime import datetime, date, time, timedelta, timezone

# Current date/time
now = datetime.now()
today = date.today()

# Create specific date/time
dt = datetime(2024, 3, 15, 14, 30, 0)

# Timezone-aware datetime (preferred over naive)
utc_now = datetime.now(timezone.utc)

# Parse a string into datetime
dt = datetime.strptime("2024-03-15", "%Y-%m-%d")

# Format a datetime into a string
dt.strftime("%Y-%m-%d %H:%M:%S")  # "2024-03-15 14:30:00"

# ISO format (preferred for parsing/serialization)
dt.isoformat()                       # "2024-03-15T14:30:00"
datetime.fromisoformat("2024-03-15T14:30:00")

# Arithmetic with timedelta
tomorrow = today + timedelta(days=1)
diff = dt2 - dt1  # returns timedelta

Progress checklist for date/time handling tasks:

• Decide if you need date, time, datetime, or timedelta
• Decide naive vs. timezone-aware (default to aware for anything crossing systems)
• Choose parsing method (strptime, fromisoformat, or third-party if formats vary)
• Perform arithmetic/comparisons using timedelta
• Format output with strftime or isoformat
• Test edge cases: DST transitions, leap years, month/year boundaries

1. Choose the right type

• date: calendar date only (year, month, day)
• time: time of day only (hour, minute, second, microsecond, tzinfo)
• datetime: combination of both — use this for almost everything
• timedelta: duration/difference between two dates or times

2. Naive vs. aware

Naive objects have no tzinfo — never compare naive and aware datetimes (raises TypeError). Always attach a timezone when the value will be stored, transmitted, or compared across systems:

Python
from datetime import timezone, timedelta

# UTC
dt = datetime.now(timezone.utc)

# Fixed offset
tz = timezone(timedelta(hours=-5))
dt = datetime.now(tz)

# Full IANA timezone support (Python 3.9+)
from zoneinfo import ZoneInfo
dt = datetime.now(ZoneInfo("America/New_York"))

3. Parsing input

• Known fixed format → strptime(s, fmt)
• ISO 8601 string → datetime.fromisoformat(s) (handles most ISO variants natively in modern Python)
• Unix timestamp → datetime.fromtimestamp(ts, tz=timezone.utc)

4. Arithmetic and comparison

Python
delta = timedelta(weeks=1, days=2, hours=3)
future = now + delta
diff = future - now          # timedelta
diff.total_seconds()         # float seconds

# Comparisons work directly
if dt1 < dt2:
    ...

5. Formatting output

Python
dt.strftime("%A, %B %d, %Y")   # "Friday, March 15, 2024"
dt.isoformat(timespec="seconds")

Example 1: Parse and reformat a date string Input: "2024-03-15" needs to become "March 15, 2024" Output:

Python
dt = datetime.strptime("2024-03-15", "%Y-%m-%d")
dt.strftime("%B %d, %Y")  # "March 15, 2024"

Example 2: Compute age in days between two dates Input: date(1990, 5, 20) and date.today() Output:

Python
delta = date.today() - date(1990, 5, 20)
delta.days  # integer number of days

Example 3: Convert a UTC timestamp to a local timezone Input: Unix timestamp 1710512400, target zone "Europe/Berlin" Output:

Python
from zoneinfo import ZoneInfo
dt_utc = datetime.fromtimestamp(1710512400, tz=timezone.utc)
dt_local = dt_utc.astimezone(ZoneInfo("Europe/Berlin"))

Example 4: Add 30 business-agnostic days to a deadline Input: datetime(2024, 1, 1), add 30 days Output:

Python
deadline = datetime(2024, 1, 1) + timedelta(days=30)  # datetime(2024, 1, 31)