Stripe·Software Engineer·Technical Phone Screen·Intermediate

IntermediatePrefer not to say

Apr 2026

Summary

Stripe coding round for a software engineer role. The problem looked like a calendar/scheduling utility but the edge cases around boundary validation made it way harder than it sounded. You also had to write your own tests, which I wasn't fully prepared for.

Questions Asked (1)

Given a date range and a weekly recurring schedule configuration, generate all valid 30-minute time slots within that range. The first and last days must be strictly validated so no slot starts before the range's start time or ends after the range's end time.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

The core logic wasn't too bad once I got the weekly schedule iteration working.

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AI HintsAI Generated

Suggested Approach

Clarify the inputs: a date range (start and end datetime) and a weekly recurring schedule (e.g., days of week with start/end times). Then iterate day by day, generating 30-minute slots that fall within the schedule for that day, and finally filter out any slots that start before the range's start or end after the range's end.

Pro tip: Emphasize that you'll handle time zones and DST explicitly, and that you'll use half-open intervals [start, end) to avoid off-by-one errors at boundaries.

1. Clarify requirements and edge cases

Ask about time zone handling, DST transitions, whether the schedule can span midnight, and whether the range boundaries are inclusive or exclusive. Confirm that slots must be exactly 30 minutes and cannot overlap the range boundaries.

2. Parse and normalize inputs

Convert the date range and weekly schedule into a consistent representation, such as UTC timestamps or local datetime objects with time zone awareness. Ensure the schedule is expanded into concrete time intervals for each day of the week.

3. Iterate over days and generate slots

For each day in the range, determine the applicable schedule intervals (if any). Within each interval, generate 30-minute slots by stepping from the interval start in 30-minute increments, ensuring the slot fits entirely within the interval.

4. Validate against range boundaries

For the first and last days of the range, strictly filter slots: a slot is valid only if its start time is >= range start and its end time is <= range end. For middle days, all slots within schedule intervals are valid.

5. Return and discuss trade-offs

Collect all valid slots into a list, sorted chronologically. Discuss trade-offs: e.g., generating all slots upfront vs. lazy evaluation, handling large ranges, and time zone complexities.

Key Points to Mention

Time zone and DST handling: use time zone-aware datetimes and consider DST transitions that may cause skipped or repeated times.
Boundary conditions: strictly enforce that no slot starts before range start or ends after range end, especially on first and last days.
Efficiency: iterate day by day and generate slots only for scheduled intervals, avoiding unnecessary checks for unscheduled days.
Data structures: use a list to store slots, and consider a set if deduplication is needed due to overlapping schedule intervals.
Edge cases: empty schedule, range shorter than 30 minutes, schedule intervals not aligned to 30-minute boundaries, and ranges spanning multiple weeks.
Testing: suggest unit tests for boundaries, DST transitions, and different schedule configurations.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.