The Streak That Died at Midnight — Python Bug Hunt

Inspired by the streak-loss support tickets every habit app drowns in.

Language: Python
Layer: Backend
Difficulty: Medium
Concepts: Time, Timezones
Modelled on: Duolingo-class apps
Visible tests: practicing today keeps the streak; yesterday keeps it, two days ago kills it; the local day respects the offset
Reward: 50 XP for a complete fix

Briefing

Inspired by the streak-loss support tickets every habit app drowns in. Two bugs conspire: the "did they practice yesterday?" check is off by one, and the local-day computation ignores the user's timezone, so evening sessions land on the wrong day.

streak.py — days are integer day-numbers; offsets are minutes east of UTC.

Bug report

BUG-STREAK · Priority: High (App Store reviews) · Reported by: growth

local_day(epoch_seconds, offset_minutes) = the user's local day number
is_alive(last_practice_day, today) — alive if last practice was today OR yesterday

Observed: practicing twice in one day KILLS the streak, and a 22:13 UTC session for a UTC+2 user is credited to the previous local day.

Logs

[streak] user practiced today -> streak reset (?!)
[streak] 22:13Z @ UTC+2 -> counted for yesterday

The code as shipped

src/habit/streak.py (editable)

SECONDS_PER_DAY = 86400

def local_day(epoch_seconds, offset_minutes):
    return epoch_seconds // SECONDS_PER_DAY

def is_alive(last_practice_day, today):
    return 0 < today - last_practice_day < 2

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