Python Time

• Use datetime.datetime with an explicit timezone, never a naive datetime for an instant in time
• Create “now” with datetime.now(UTC) from datetime, not datetime.utcnow() and not datetime.now()
• Store and compare instants in UTC; convert to a local zone only when formatting for a human
• Use date for a calendar day and timedelta for a duration, do not encode those as strings or ints of seconds unless an external protocol requires it
• Parse incoming timestamps with an explicit format or fromisoformat, and reject values with no offset
• Freeze time in tests with a clock injected at the boundary, not by patching every call site

Good:

from datetime import UTC, datetime

def recorded_at() -> datetime:
    return datetime.now(UTC)

Bad:

from datetime import datetime

def recorded_at() -> datetime:
    return datetime.now()