DST Boundaries and the Missing Hour

Concepts covered: sqlDateTrunc

Most companies do not align their fiscal year with the calendar year. Apple's fiscal year ends in September; many retailers end in January or February; the US government ends in September. A query that uses DATE_TRUNC('year', date) for fiscal reporting is wrong for any company except those with calendar-year fiscal alignment. Knowing the patterns for fiscal calendars and business-day windows before the interviewer asks is what tells the room you've reported against a real company's books. Fiscal year arithmetic Two patterns for fiscal-year computation. Pattern one: arithmetic shift. If the fiscal year starts in April, subtract three months from the date before truncating; the result is the fiscal-year-aligned year. txn_date - INTERVAL '3 months' converts the dates, then DATE_TRUNC('year',

About This Interactive Section

This section is part of the Date Arithmetic: Intermediate lesson on DataDriven, a free data engineering interview prep platform. Each section includes explanations, worked examples, and hands-on code challenges that execute in real time. SQL queries run against a live database. Python runs in a sandboxed Docker container. Data modeling problems validate against interactive schema canvases. All content is framed around what data engineering interviewers actually test at companies like Meta, Google, Amazon, Netflix, Stripe, and Databricks.

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