[Alicepoltora]

Problem

_format_created_at() in UsageMetricsService used datetime.utcnow() to produce the fallback ISO 8601 timestamp when created_at is None. datetime.utcnow() was deprecated in Python 3.12 and returns a naive datetime (no timezone). The returned string has no offset (e.g. 2024-01-01T12:00:00.123456), which is inconsistent with every other timestamp in Studio (all others are +00:00-aware).

Fix

Replace datetime.utcnow().isoformat() with datetime.now(timezone.utc).isoformat() so the fallback string carries a timezone offset (+00:00).

Tests

• tests/unit/test_usage_metrics_service.py: 2 new regression tests:
  • test_format_created_at_returns_utc_isoformat — fails on main (naive datetime has no + offset), passes with the fix
  • test_format_created_at_returns_original_datetime_isoformat — ensures datetime objects retain their original format

Summary by CodeRabbit

• Bug Fixes
  • Automatically generated usage-metric timestamps now include an explicit UTC timezone offset, making their timezone clear. Existing timestamps retain their previous formatting.
• Tests
  • Added coverage to verify the timezone offset for generated timestamps and confirm existing timestamps are formatted consistently.

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• backend/services/usage_metrics_service.py
• tests/unit/test_usage_metrics_service.py

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