datetime & Timezone Handling

The Four Core Types

from datetime import date, time, datetime, timedelta

date(2026, 9, 22)                  # calendar day only
time(14, 30, 5)                    # wall-clock time
datetime(2026, 9, 22, 14, 30)      # date + time
timedelta(days=7, hours=3)         # a duration — the only thing you subtract

due = date.today() + timedelta(days=14)
opened = datetime(2026, 1, 1, 9)
closed = opened + timedelta(minutes=45)
elapsed = closed - opened          # timedelta result

timedelta supports days, seconds, microseconds — combining is normalized (1 day + 25 hours = 2 days + 1 hour). Parsing and formatting use format codes: datetime.strptime(s, "%Y-%m-%dT%H:%M:%SZ") and .isoformat() for output. date.today() and datetime.now() read the system local clock; anything serialized to a log or API should be timezone-aware first.

Timezones with zoneinfo

Since Python 3.9 the stdlib has IANA time zones:

from datetime import datetime, timezone
from zoneinfo import ZoneInfo

utc = datetime.now(timezone.utc)                 # always store instants in UTC
tokyo = utc.astimezone(ZoneInfo("Asia/Tokyo"))   # convert for display
local = datetime(2026, 9, 22, 9, 0, tzinfo=ZoneInfo("America/New_York"))

Rules that keep you safe:

• Aware datetimes carry tzinfo; naive ones do not. Subtracting aware from naive raises TypeError
• astimezone() with no argument converts to the system local zone
• On Windows (and slim images), install tzdata — the IANA database is not bundled
• Legacy pytz requires localize()/ normalize(); zoneinfo attaches correctly via tzinfo=

Pitfalls and Interview Angles

DST edges. A local wall time can occur twice (fall-back) or not at all (spring-forward). The fold attribute (0 or 1) disambiguates the repeated hour when converting.

Deprecated shortcuts. datetime.utcnow() and utcfromtimestamp() return naive values misread as local — deprecated since 3.12; use datetime.now(timezone.utc).

Monotonic questions. Measuring elapsed time with datetime.now() breaks when the clock is adjusted; use time.perf_counter() or time.monotonic().

Interview angle: store instants as UTC-aware, convert to user zones only at the presentation edge, format with ISO 8601, and explain why %z/%Z behave differently across platforms (offset versus abbreviation).