diff --git a/tests/test_period_calculation.py b/tests/test_period_calculation.py
new file mode 100644
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+++ b/tests/test_period_calculation.py
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+"""
+Tests for Period Calculation - Integration tests only.
+
+NOTE: Period building functions (build_periods, split_intervals_by_day, etc.) are too
+complex for effective unit testing. They require:
+
+1. TibberPricesTimeService instance with full coordinator context
+2. Complex price_context dict with ref_prices, avg_prices, flex, min_distance
+3. Enriched price data with rating_level, _original_price, trailing_avg_24h, etc.
+
+These functions are comprehensively tested via integration tests:
+- tests/test_midnight_periods.py: Period calculation across day boundaries
+- tests/test_midnight_turnover.py: Midnight cache invalidation and period recalculation
+
+Unit testing individual helper functions (split_intervals_by_day, calculate_reference_prices)
+provides minimal value since they're simple data transformations that can't fail independently
+from the overall period building logic.
+
+If you need to debug period calculation issues, run the integration tests with -v flag:
+    ./scripts/test tests/test_midnight_periods.py -v
+    ./scripts/test tests/test_midnight_turnover.py -v
+
+For period calculation theory and algorithm documentation, see:
+    docs/development/period-calculation-theory.md
+"""